xwayland/glamour-gbm: Handle DRM_FORMAT_MOD_INVALID gracefully

The compositor may send DRM_FORMAT_MOD_INVALID instead of a list of
modifiers for various reasons. Handle this gracefully by ignoring it.

Without this, if a compositor would send DRM_FORMAT_MOD_INVALID, it'd
result in empty windows provided by Xwayland.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for wip/wayland-dma-buf-only-invalid in 7 minutes and 46 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Docker Image
passed #642646
debian-testing

00:00:41

 
  Build And Test
passed #642647
autotools-build-and-test

00:07:01

passed #642648
meson-build-and-test

00:01:48