cogl: Enable EGL_IMG_context_priority
This is probably a good idea, though I've not gone through and tested it yet. The theory is that since we're closest to the display pipeline, we should hint to the system that our rendering is critical to presentation framerate. If we have a standard testcase for performance in terms of consistently hitting framerate, I'm all ears.
Note that this only adds the feature for (what I think is) the primary EGL context. In a PRIME setup we might want to enable this for all GPUs.