Surely, we should fail to init GBM backend if "GL_OES_EGL_image" is
missing.
This seems to have been lost with commit 1545e2dba ("xwayland: Decouple
GBM from glamor").
Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>