Revert "vo_x11: remove this video output"

This reverts commit d11184a.

Unfortunately, there was a lot of unexpected resistance.

Do note that this is still extremely slow, crappy, etc.

Note that vo_x11.c was further edited. Compared to the removed vo_x11.c,
an additional ~200 lines of code was removed in order to simplify it. I
tried to strip it down as much as possible. In particular, support for
odd non-32 bit formats (24, 16, 15, 8 bit) is dropped.

Closes #2300.

DOCS/man/vo.rst

@@ -75,6 +75,12 @@ Available video output drivers are:
         not responding quickly enough if video FPS is close to or higher than
         the display refresh rate.
 
+``x11`` (X11 only)
+    Shared memory video output driver without hardware acceleration that works
+    whenever X11 is present.
+
+    .. note:: This is a fallback only, and should not be normally used.
+
 ``vdpau`` (X11 only)
     Uses the VDPAU interface to display and optionally also decode video.
     Hardware decoding is used with ``--hwdec=vdpau``.

TOOLS/old-makefile

@@ -93,7 +93,8 @@ SOURCES-$(VAAPI_VPP)            += video/filter/vf_vavpp.c
 SOURCES-$(VAAPI_GLX)            += video/out/opengl/hwdec_vaglx.c
 SOURCES-$(VAAPI_X_EGL)          += video/out/opengl/hwdec_vaegl.c
 
-SOURCES-$(XV)                   += video/out/x11_common.c video/out/vo_xv.c
+SOURCES-$(X11)                  += video/out/vo_x11.c video/out/x11_common.c
+SOURCES-$(XV)                   += video/out/vo_xv.c
 SOURCES-$(WAYLAND)              += video/out/vo_wayland.c \
                                    video/out/wayland_common.c \
                                    video/out/wayland/buffer.c \

video/out/vo.c

@@ -47,6 +47,7 @@
 #include "osdep/io.h"
 #include "osdep/threads.h"
 
+extern const struct vo_driver video_out_x11;
 extern const struct vo_driver video_out_vdpau;
 extern const struct vo_driver video_out_xv;
 extern const struct vo_driver video_out_opengl;
@@ -90,6 +91,9 @@ const struct vo_driver *const video_out_drivers[] =
 #endif
 #if HAVE_VAAPI
     &video_out_vaapi,
+#endif
+#if HAVE_X11
+    &video_out_x11,
 #endif
     &video_out_null,
     // should not be auto-selected