The Egl backend opens the drm device and creates a gbm_device from it.
From that it creates the EGL device. The EGL device outlives the EGL
backend (e.g. for compositor restart). So far the EGL backend destroyed
the gbm_device when going down which mesa did not like at all when a
new context got created on that EGL device.
Thus the ownership of gbm_device is also passed to the Platform so that
the compositor can be restarted.
This makes the TestSceneOpenGL no longer crash during the restart
compositor test.
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