struct gl_config cleanup

This struct is just bloated and icky, either the state that's in it doesn't belong or it's in the most useless possible representation. This series takes it from 184 to 104 bytes (on all arches), with knock-on benefits to gl_context and gl_framebuffer. Eventually I want it to basically be identical to st_visual, and the next series after this one will mostly be about using pipe_formats instead of bits/mask/shift/etc.