misc cleanups
Here's a bunch of minor cleanups, mostly related to ifdefs.
In particular, we test for a lot of defines from GL/gl.h
. But most programs use GLAD now, which always define these. So they are useless.
There's a few more cases of #if(n)def GL_foo
left over in the tree. These are either from programs not (yet?) using GLAD, or from common code where we the including program might not include GLAD. I didn't do the proper analysis of the latter case to know if some of them could be removed, that's an exercise left for the future.