GLX: Use the sending client for looking up XID's

When GlxGetXIDMap looks up an unknown XID, it will now look up a vendor based
on the screen number for the XID and the client that sent the current request.

In GlxGetXIDMap, if the XID is for a regular X window, then it won't be in the
(XID -> vendor) mapping, so we have to look up a vendor by screen number.

With this change, GlxGetXIDMap will use the (screen -> vendor) map for
whichever client sent the current request, instead of using the global
(screen -> vendor) map.

Since GlxGetXIDMap doesn't take a ClientPtr argument, GlxDispatchRequest will
store the client for the current request in a global variable. That way, the
ABI for GLXVND doesn't need to change.

v2: Fix an error check in GlxDispatchRequest.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 8b67ec7c)