xwayland: Decouple GBM from glamor

This takes all of the gbm related code in wayland-glamor.c and moves it
into it's own EGL backend for Xwayland, xwayland-glamor-gbm.c.
Additionally, we add the egl_backend struct into xwl_screen in order to
provide hooks for alternative EGL backends such as nvidia's EGLStreams.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>