libGLX: Add a function to check if a vendor library supports a screen.

Created by: kbrenneman

This adds a function to the ABI to determine whether a vendor library will work with a particular X screen.

Once we've got an indirect rendering library as part of libglvnd, other vendor libraries will only have to implement direct rendering. For indirect rendering, they'll be able to just return false and let the indirect rendering library deal with it instead.

Note that this only affects the vendor library selected using the x11glvnd extension. If the user overrides the vendor name with the __GLX_VENDOR_LIBRARY_NAME environment variable, then it'll use that vendor without checking for support.