shaders macro: Expose top level GLSL constant

[tnibler]

It would be handy to have rust constants generated from GLSL constants (for example static array sizes, const uint MAX_FOOS = 32;), so you can assert at compile or runtime that your stuff and buffer sizes match up with what's declared in the shaders. I know they are already resolved and used for the types of struct array members, it's just sort of annoying to get at the sizes that way.

It's a small change but I couldn't figure out how the generated code for parsing and everything in 5 minutes, but I could look into it when I have more time.

Thanks :)