Preprocessor for glsl-new

[lachlansneff]

I threw together a quick, partial implementation of the glsl preprocessor (notably doesn't support #if or #elif because I didn't want to implement constant expression evaluation) for bevy (here) because the glsl-new frontend doesn't have one. I'm happy to donate this to naga if it's wanted as a starting point.

[pjoe]

Initial code in #238

[fintelia]

I think this is resolved?

[kvark]

@pjoe had concerns about the way we have it currently. I think the question still remains open, although it would be great to spell out all the details here.

[kvark]

#238 got merged