Replace x11glvnd with GLX_EXT_libglvnd extension
Created by: kbrenneman
This replaces the x11glvnd X extension with a new GLX extension, based on the suggestions in pull request #76.
To look up the vendor name for a screen, libGLX now sends a glXQueryServerString request with a new GLX_VENDOR_NAMES_EXT enum.
To look up the screen number for a drawable, it'll send a glXGetDrawableAttributes request, and the server will add a GLX_SCREEN attribute to the reply.