This is based on an out-of-tree module written by Kyle:
https://github.com/kbrenneman/libglvnd/tree/server-libglx
I (ajax) did a bunch of cosmetic fixes, ported it off xfree86 API,
added request length checks, and fixed a minor bug or two.
v2: Use separate functions to set/get a context tag's private data, and
call the backend's MakeCurrent when a client disconnects to unbind the
context. (Kyle Brenneman)
Signed-off-by: Adam Jackson <ajax@redhat.com>