If Xwayland's EGLstream backend supports hardware acceleration with the
NVIDIA closed-source driver, the GLX library also needs to be one
shipped by NVIDIA, that's what GLVND is for.
Add a new member to the xwl_screen that the backend can optionally set
to the preferred GLVND vendor to use.
If not set, "mesa" is assumed.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>