glx: Implement GLX_EXT_get_drawable_type

Merged Adam Jackson requested to merge ajax/xserver:glx-ext-get-drawble-type into master

Trivial extension to let the client query whether this is a window pixmap or pbuffer. Mostly for Mesa's convenience when setting up drawable state, but plausibly useful for apps and middleware as well.

Upstream OpenGL Registry merge request:

Merge request reports