GLX: Add a per-client vendor mapping.

Each client now has its own (screen, vendor) mapping.

Currently, it's just a copy of the global mapping, but later changes will allow
it to change.

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 37a36a6b)