cogl: Clean up private feature flags and GLSL builtin handling of same

GNOME/mutter!995
6 jobs for !995 with cogl-less-fixed-api-usage in 11 minutes and 16 seconds
detached
Status Job ID Name Coverage
  Review
passed #557595
check-commit-log

00:00:24

 
  Build
passed #557596
build-mutter

00:03:26

passed #557597
build-without-native-backend-and-wayland

00:04:49

 
  Test
passed #557599
can-build-gnome-shell

00:03:14

passed #557618
test-mutter

00:02:47

failed #557598
test-mutter

00:04:57