backend/native: Use high priority contexts for secondary GPUs

Ensure the context requests EGL_CONTEXT_PRIORITY_HIGH_IMG, and
that we use cogl_egl_create_context() to ensure that is honored.

!923
5 jobs for !923 with wip/carlosg/drop-caps in 5 minutes and 36 seconds (queued for 1 second)
latest detached
Status Name Job ID Coverage
  Review
passed check-commit-log #498535

00:00:38

 
  Build
passed build-mutter #498536

00:01:44

passed build-without-native-backend-and-wayland #498537

00:02:08

 
  Test
passed can-build-gnome-shell #498539

00:01:26

passed test-mutter #498538

00:02:50