Support libpng with no iTXt chunk support

[AbigailBuccaneer]

libpng's support for the iTXt chunk ('international textual data') is
optional, and off-by-default before libpng 1.4, which causes a compile
error. This writes an iTXt chunk only if supported, and writes a tEXt
chunk otherwise. (A tEXt chunk is identical but specified to contain
Latin-1 text instead of UTF-8, and has no language tag.)

[Deledrius]

Thank you for submitting this fix!

The switch to using iTXt rather than tEXt chunks happened pretty late in the work on this feature, and I wasn't aware of the version limitations. I don't see it in the libpng 1.2 documentation for these chunks at all. Do you have a reference for this information we can look to in the future?

I'll be able to give this a test in the next few days. Thanks again!

[AbigailBuccaneer]

That's the documentation for the PNG file format itself, not for libpng. The libpng manual says:

Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6;
however, iTXt support was not enabled by default.
...
Support for the iTXt chunk has been enabled by default as of
version 1.2.41.
...
    Note that the itxt_length, lang, and lang_key
    members of the text_ptr structure only exist when the
    library is built with iTXt chunk support.  Prior to
    libpng-1.4.0 the library was built by default without
    iTXt support. Also note that when iTXt is supported,
    they contain NULL pointers when the "compression"
    field contains PNG_TEXT_COMPRESSION_NONE or
    PNG_TEXT_COMPRESSION_zTXt.

pngconf.h from libpng 1.2.59 [source tarball] says:

/* The size of the png_text structure changed in libpng-1.0.6 when
 * iTXt support was added.  iTXt support was turned off by default through
 * libpng-1.2.x, to support old apps that malloc the png_text structure
 * instead of calling png_set_text() and letting libpng malloc it.  It
 * will be turned on by default in libpng-1.4.0.
 */

#if defined(PNG_1_0_X) || defined (PNG_1_2_X)
#  ifndef PNG_NO_iTXt_SUPPORTED
#    define PNG_NO_iTXt_SUPPORTED
#  endif
#  ifndef PNG_NO_READ_iTXt
#    define PNG_NO_READ_iTXt
#  endif
#  ifndef PNG_NO_WRITE_iTXt
#    define PNG_NO_WRITE_iTXt
#  endif
#endif

[Deledrius]

Oh, I was looking in the wrong place the whole time. 😳

[zrax]

This can throw, since ST::string expects to hold UTF-8 data. Instead, the char_buffer should be stored off separately, and should point to either the UTF-8 data or Latin-1 data to be written by libpng.

[AbigailBuccaneer]

@zrax LGTY?

[Deledrius]

Looks great. 👍