compositor: Don't emit size-changed when only position changes

Waking up gnome-shell and triggering JavaScript listeners of
`size-changed` every time a window was only moved was wasting a lot
of CPU.

This cuts the CPU requirement for dragging windows by around 22%.

GNOME/mutter!568
4 jobs for !568 with size-changed-not in 6 minutes and 33 seconds
detached
Status Job ID Name Coverage
  Review
passed #323429
check-commit-log

00:00:29

 
  Build
passed #323430
build-mutter

00:03:04

 
  Test
passed #323432
can-build-gnome-shell

00:01:49

passed #323431
test-mutter

00:02:59