Implement font lookup in ext/gd

Description

As is, libgd supports a somewhat complex font lookup for gdImageStringFT() (and friends). You can actually pass a semicolon delimeted list of fonts, and each font can be an absolute or relative path to a font file, or a suffix to any of the default font paths, which are hard-coded for different platforms, but which also can be overridden via the environment variable GDFONTPATH. As is, we pass the $font_file argument of imagettftext() and friends directly to gdImageStringFT(), so libgd does all the heavy work to actually find the desired font.

However, this does not (necessarily) work for ZTS builds when relative paths are given, since external libgd is not aware of the VCWD (while bundled libgd is) (see #17366). In addition, changing a global enviroment variable doesn't fit well to our ZTS model (see https://github.com/php/php-src/pull/16565#issuecomment-2665494265). And there are also some minor issues with the libgd implementation (e.g. https://github.com/libgd/libgd/issues/952).

In my opinion, the font lookup in libgd should have better been split from the actual drawing functions, what would have allowed us to implement our own font lookup in the first place, properly catering to VCWD and what not (e.g. fontconfig support, which is implemented in libgd, but not supported from ext/gd). Now this is moot, but I still suggest to implement our own font lookup in ext/gd (basically re-implementing what libgd does), and then to pass only a single absolute font path to gdImageStringFT(). This would allow to compensate the current incompatibilites between external and bundled font lookup (partially paving the way to unbundle libgd), and would also allow us to make further improvements (especially fontconfig comes to mind), with having to deal with the somewhat messy way that is implemented in external libgd (see gdImageStringFTEx()) (bundled libgd doesn't even support that).

Start with the font-related entry points in ext/gd, especially imagettftext() and its related functions, and compare their current handoff to gdImageStringFT() with libgd's gdImageStringFTEx() lookup behavior. Done means font lookup is implemented in ext/gd, handles the stated path and environment cases with VCWD considerations, and passes a single absolute font path to gdImageStringFT().