xwayland: Set GLVND driver based on GBM backend name
With the GBM backend becoming usable with different drivers such as NVIDIA, set the GLVND vendor to the same value as the GBM backend name.
Mesa implementation however returns "drm" so we need to special case this value - Basically, for anything other than "drm" we simply assume that the GBM backend name is the same as the vendor.
This is preparation work for !751 (merged)
cc: @cubanismo, @daenzer
Edited by Olivier Fourdan