glx: Implement GLX_EXT_get_drawable_type

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:

https://github.com/KhronosGroup/OpenGL-Registry/pull/425
4 jobs for glx-ext-get-drawble-type in 5 minutes and 35 seconds (queued for 2 seconds)