glx: Implement the libglvnd interface.

glx: Implement the libglvnd interface.

With reference to the libglvnd branch:

https://cgit.freedesktop.org/mesa/mesa/log/?h=libglvnd

This is a squashed commit containing all of Kyle's commits, all but two
of Emil's commits (to follow), and a small fixup from myself to mark the
rest of the glX* functions as _GLX_PUBLIC so they are not exported when
building for libglvnd. I (ajax) squashed them together both for ease of
review, and because most of the changes are un-useful intermediate
states representing the evolution of glvnd's internal API.

Co-author: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>