clutter/frame-clock: Rename "before_frame" to "update"

This callback now represents the update stage of a potential frame,
so name it appropriately.

This callback allows users the opportunity to perform some operatons
at regular frame timing interval without being delayed by GPU load,
number of available frame buffers or pending presentation feedback.

!1241
6 jobs for !1241 with more-frequent-updates in 9 minutes and 46 seconds (queued for 1 second)
latest detached
Status Name Job ID Coverage
  Review
passed check-commit-log #914039

00:00:15

 
  Build
passed build-mutter #914040

00:05:00

passed build-without-native-backend-and-wayland #914042

00:02:14

passed build-without-opengl-and-glx #914041

00:04:07

 
  Test
passed can-build-gnome-shell #914044

00:02:18

passed test-mutter #914043

00:04:45