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.
3 jobs for wip/wayland-dma-buf-only-invalid in 11 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #606017
debian-testing

00:06:00

 
  Build And Test
passed #606018
autotools-build-and-test

00:05:06

passed #606019
meson-build-and-test

00:01:52