gl-renderer: Create a high priority context

EGL_IMG_context_priority allows the client to request that their
rendering be considered high priority. For ourselves, this is important
as we are interactive and any delay in our rendering causes input-output
jitter; a less than smooth user interactive. So if the driver supports
setting the context priority, try and create our EGLContext as high
priority. The driver may reject our request due to system restrictions,
in which case it will fallback to normal priority, but if successful it
will reschedule our rendering and all of its dependencies to execute
earlier, especially important when the GPU is being hogged by background
clients.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>