Merge SIXEL branch.

Squashed commit of the following:

commit 6ebc3fe
Author: topcat001 <anindya49@hotmail.com>
Date:   Sun Aug 20 16:09:51 2023 -0700

    Remove redundant {}.

commit 6f013fc
Author: topcat001 <anindya49@hotmail.com>
Date:   Sun Aug 20 16:02:15 2023 -0700

    Revert "Do not defer redraw if it is just the status line (will need to do more here I"

    This reverts commit 0a15bbf.

commit e6322b4
Author: topcat001 <anindya49@hotmail.com>
Date:   Sun Aug 20 15:46:59 2023 -0700

    Fix placeholder label and clean up.

commit 5896ac5
Merge: ad98233 e3a8b84
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Fri Aug 18 17:00:03 2023 +0100

    Merge branch 'master' into sixel

commit ad98233
Author: topcat001 <anindya49@hotmail.com>
Date:   Tue Aug 15 13:57:08 2023 -0700

    Better text placeholder.

commit 312d832
Merge: 14b8b52 3d93b0c
Author: topcat001 <anindya49@hotmail.com>
Date:   Tue Aug 15 13:39:22 2023 -0700

    Merge remote-tracking branch 'origin/master' into sixel

commit 14b8b52
Merge: 4baf764 fda3937
Author: topcat001 <anindya49@hotmail.com>
Date:   Sat Jul 22 17:29:10 2023 -0700

    Merge branch 'master' into sixel

commit 4baf764
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Wed Jun 21 07:43:53 2023 +0100

    Both files can go on one line.

commit 4c92acf
Author: topcat001 <anindya49@hotmail.com>
Date:   Sat Jun 17 17:53:01 2023 -0700

    Merge topcat001/tmux/sixel.

commit 6794fac
Merge: 7b85f5a f41c536
Author: topcat001 <anindya49@hotmail.com>
Date:   Sat Jun 17 17:21:02 2023 -0700

    Merge remote-tracking branch 'origin/master' into sixel

commit 7b85f5a
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Jun 8 12:55:03 2023 +0100

    Do not require passthrough for SIXEL.

commit a6ee55e
Merge: 6da391f fe385b1
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Jun 8 12:19:55 2023 +0100

    Merge branch 'master' into sixel

commit 6da391f
Merge: 0d71e58 0eb5d25
Author: topcat001 <anindya49@hotmail.com>
Date:   Sat May 20 17:05:55 2023 -0700

    Merge branch 'master' into sixel

commit 0d71e58
Merge: 64368a1 fbe6fe7
Author: topcat001 <anindya49@hotmail.com>
Date:   Sat Apr 29 17:32:07 2023 -0700

    Merge branch 'master' into sixel

commit 64368a1
Merge: c630a56 22eb033
Author: topcat001 <anindya49@hotmail.com>
Date:   Thu Mar 30 14:21:09 2023 -0700

    Merge branch 'master' into sixel

commit c630a56
Merge: 34c96c4 aaa043a
Author: topcat001 <anindya49@hotmail.com>
Date:   Thu Nov 10 18:53:01 2022 -0800

    Merge branch 'master' into sixel

commit 34c96c4
Merge: 2a1e16a 50f4e0f
Author: topcat001 <anindya49@hotmail.com>
Date:   Sat Nov 5 18:05:36 2022 -0700

    Merge branch 'master' into sixel

commit 2a1e16a
Merge: a82f14c d001a94
Author: topcat001 <anindya49@hotmail.com>
Date:   Thu Oct 27 16:01:35 2022 -0700

    Merge branch 'master' into sixel

commit a82f14c
Merge: 742c063 f7b30ed
Author: topcat001 <anindya49@hotmail.com>
Date:   Sun Aug 28 13:43:07 2022 -0700

    Merge branch 'master' into sixel

commit 742c063
Merge: 906c92a 87b248f
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Fri Apr 1 10:14:15 2022 +0100

    Merge branch 'master' into sixel

commit 906c92a
Merge: 6680a02 138ffc7
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Wed Dec 8 10:37:33 2021 +0000

    Merge branch 'master' into sixel

commit 6680a02
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Oct 7 13:59:08 2021 +0100

    Fix build.

commit ebd2c58
Merge: 90dc051 fed7b29
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Oct 7 13:19:48 2021 +0100

    Merge branch 'master' into sixel

commit 90dc051
Merge: a282439 4694afb
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Feb 20 20:37:32 2020 +0000

    Merge branch 'master' into sixel

commit a282439
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Jan 30 09:12:53 2020 +0000

    Add missing declarations.

commit 3a741aa
Merge: 40ad010 339832b
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Jan 30 09:11:01 2020 +0000

    Merge branch 'sixel-passthrough' into sixel

commit 339832b
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Jan 30 09:04:51 2020 +0000

    Bad merge.

commit 92ed9fc
Merge: 5bb0754 32be954
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Jan 30 09:03:38 2020 +0000

    Merge branch 'master' into sixel-passthrough

commit 40ad010
Merge: dd3c72f 61b075a
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Sun Jan 12 20:03:41 2020 +0000

    Merge branch 'master' into sixel

commit 5bb0754
Merge: 7c033a7 54efe33
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Wed Dec 18 20:24:42 2019 +0000

    Merge branch 'master' into sixel-passthrough

commit dd3c72f
Merge: 1a0e5fe 54efe33
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Wed Dec 18 20:24:26 2019 +0000

    Merge branch 'master' into sixel

commit 1a0e5fe
Merge: cf071ff 15d7e56
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Tue Dec 10 16:34:11 2019 +0000

    Merge branch 'master' into sixel

commit cf071ff
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Mon Dec 9 15:41:56 2019 +0000

    Remove images when reflow happens.

commit 2006b7a
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Dec 5 09:27:15 2019 +0000

    More invalidation of images.

commit b642eac
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Dec 5 09:11:24 2019 +0000

    Redraw and scroll images and part of invalidating them.

commit 7566e37
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Dec 5 08:51:24 2019 +0000

    Call sixel_scale with the right number of arguments.

commit 62c0280
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Dec 5 08:48:58 2019 +0000

    Correctly remove when not visible.

commit 86c5098
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Dec 5 08:32:25 2019 +0000

    Add helpers to scroll image up and a flag to copy the colours.

commit 49f2f0a
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Dec 5 00:02:55 2019 +0000

    Store images, currently at most 10.

commit 3aebcc6
Merge: 146ee3f 92ecd61
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Wed Dec 4 19:27:16 2019 +0000

    Merge branch 'master' into sixel

commit 7c033a7
Merge: 0a15bbf 92ecd61
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Wed Dec 4 12:41:09 2019 +0000

    Merge branch 'master' into sixel-passthrough

commit 146ee3f
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Sat Nov 30 09:47:53 2019 +0000

    Don't write image as text yet.

commit 0a15bbf
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Sat Nov 30 09:15:35 2019 +0000

    Do not defer redraw if it is just the status line (will need to do more here I
    think).

commit a5b1e20
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 14:20:22 2019 +0000

    Add a flag to disable blocking while sending a SIXEL image (turned off when the
    buffer hits 0 size).

commit 968382a
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 12:35:18 2019 +0000

    Pass through SIXEL DCS sequences (treat similarly to the passthrough escape
    sequence) if it appears the terminal outside supports them.

commit b1904c9
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Sat Nov 30 09:17:18 2019 +0000

    Store SIXELs as a box for the moment.

commit 5d8dbcd
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Sat Nov 30 09:15:35 2019 +0000

    Do not defer redraw if it is just the status line (will need to do more here I
    think).

commit 0c999a4
Merge: 28961dd 866b053
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Fri Nov 29 18:54:09 2019 +0000

    Merge branch 'master' into sixel

commit 28961dd
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 14:24:57 2019 +0000

    Add an image.

commit d2e3f3c
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 14:20:22 2019 +0000

    Add a flag to disable blocking while sending a SIXEL image (turned off when the
    buffer hits 0 size).

commit e01df67
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 13:21:40 2019 +0000

    Crop and scale images as needed when drawing them.

commit e24acc0
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 12:38:02 2019 +0000

    Simple SIXEL parse and modify API.

commit b34111b
Author: Nicholas Marriott <nicholas.marriott@gmail.com>
Date:   Thu Nov 28 12:35:18 2019 +0000

    Pass through SIXEL DCS sequences (treat similarly to the passthrough escape
    sequence) if it appears the terminal outside supports them.

Author: nicm

Makefile.am

@@ -218,6 +218,11 @@ if HAVE_UTF8PROC
 nodist_tmux_SOURCES += compat/utf8proc.c
 endif
 
+# Enable sixel support.
+if ENABLE_SIXEL
+dist_tmux_SOURCES += image.c image-sixel.c
+endif
+
 if NEED_FUZZING
 check_PROGRAMS = fuzz/input-fuzzer
 fuzz_input_fuzzer_LDFLAGS = $(FUZZING_LIBS)

configure.ac

@@ -449,6 +449,16 @@ if test "x$enable_cgroups" = xyes; then
 	fi
 fi
 
+# Enable sixel support.
+AC_ARG_ENABLE(
+	sixel,
+	AS_HELP_STRING(--enable-sixel, enable sixel images)
+)
+if test "x$enable_sixel" = xyes; then
+	AC_DEFINE(ENABLE_SIXEL)
+fi
+AM_CONDITIONAL(ENABLE_SIXEL, [test "x$enable_sixel" = xyes])
+
 # Check for b64_ntop. If we have b64_ntop, we assume b64_pton as well.
 AC_MSG_CHECKING(for b64_ntop)
 AC_LINK_IFELSE([AC_LANG_PROGRAM(