xwayland: Set the vendor name for GLX_EXT_libglvnd

Without this the client library will flail around looking for a default
provider, probably one named "indirect", and that defeats the point of
having the EGL provider for direct context support in the first place.

This assumes that "mesa" will work, of course, and in general it should.
Mesa drivers will DTRT through the DRI3 setup path, and if our glamor is
atop something non-Mesa then you should fall back to llvmpipe like 1.20.
In the future it might be useful to differentiate the vendor here based
on whether glamor is using gbm or streams.

Fixes: xorg/xserver#1032
4 jobs for xwayland-glvnd in 6 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #2977102
debian-testing

00:01:03

 
  Build And Test
passed #2977103
autotools

00:05:29

passed #2977104
meson

00:02:11

passed #2977105
meson-noglamor

00:01:24