clutter/cogl: Query pending_swaps instead of counting them

Cogl was already counting them internally with the `pending_frame_infos`
queue. This allows us to remove the `pending_swaps` counter.

GNOME/mutter!770
5 jobs for !770 with count-pending-frame-infos in 9 minutes and 10 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #453730
check-commit-log

00:00:21

 
  Build
passed #453731
build-mutter

00:05:59

passed #453732
build-without-native-backend

00:01:29

 
  Test
passed #453734
can-build-gnome-shell

00:01:11

passed #453733
test-mutter

00:02:49