xwayland/glamour-gbm: Handle DRM_FORMAT_MOD_INVALID gracefully

Merged Jonas Ådahl requested to merge jadahl/xserver:wip/wayland-dma-buf-only-invalid into master

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.

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