cogl: Use EGL_IMG_context_priority if available

We're the context closest to the display, so we should take priority
over other clients.

v2: Warn if we asked for a high-priority context and didn't get one.

GNOME/mutter!454
3 jobs for !454 with context-priority in 5 minutes and 18 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #276573
check-commit-log

00:00:49

 
  Build
passed #276574
build-mutter

00:01:29

 
  Test
passed #276575
test-mutter

00:02:58