xwayland/glamor-gbm: Add xwl_glamor_gbm_post_damage hook

It flushes any pending drawing to the kernel, to make sure it'll be visible to the Wayland server.

Without this, it was possible for the Wayland server to process surface commits before Xwayland got around to flushing the corresponding drawing, which would result in stale or even completely random window contents being visible.

Closes: #951 (closed)

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