GLX: Add a function to change a clients vendor list.

Add a new function, GlxServerExports::setClientScreenVendor, which will change
the vendor that handles GLX requests for a screen, but only for requests from
a specific client.

v2: Increment the GLXVND minor version number.
v3: Note the GLXVND version requirement for setClientScreenVendor.

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 56c0a71f)