clutter: More accurate & detailed CLUTTER_SHOW_FPS

Redesigned CLUTTER_SHOW_FPS printing to be more accurate, more detailed
and more efficient.

The timer is removed. FPS is now reported to two decimal places, which is
handy for comparing with your monitor refresh rate. And the render time in
milliseconds is now also displayed.

GNOME/mutter!154
3 jobs for !154 with better-show-fps in 11 minutes and 27 seconds (queued for 19 minutes and 29 seconds)
latest detached
Status Job ID Name Coverage
  Review
passed #230288
check-commit-log

00:06:29

 
  Build
passed #230289
build-mutter

00:02:37

 
  Test
passed #230290
test-mutter

00:02:20