Re-enable RXML and purge libxml

[huwpascoe]

Fixed rxml to work with griffin.c

Related Issues

#7419

[orbea]

Would you mind elaborating more on the point of this PR? What is libxml2 even used for in RetroArch?

[huwpascoe]

libxml2 isn't used for anything, it's been commented out with #ifdef 0 for a while it seems.
rxml is needed for the linked issue.

[orbea]

Rather, why was libxml added in the first place?

[huwpascoe]

I think maybe it was for opengl xml shaders or something like that?

[orbea]

Sounds plausible, thanks for satisfying my curiosity.

[markwkidd]

GLSL XML shaders I guess? https://docs.libretro.com/development/shader/xml-shaders/

[orbea]

Just for reference that page says.

XML shaders as a whole are deprecated in RetroArch, and will not be selectable in RGUI. You can still use them via video_shader config option. To use GLSL in RetroArch, the new GLSL shaders format is used, which mirrors the Cg shaders implementation quite well. The new GLSL shaders only support modern style, no fixed function. To convert XML shaders into straight GLSL, see GLSL shaders.

@hizzlekizzle Is it worth supporting XML shaders anymore? I imagine most shaders have been converted for glsl and slang by now?

[hizzlekizzle]

@orbea No, zero point AFAIK. I'm not sure if the "game-aware" shader functionality was ever added to the modern shader backends, but they were only ever proofs of concept anyway.

[huwpascoe]

I'm not going to be dealing with expat, sorry. This is just for #7419 if someone else wants to improve xml support later they can.