compositor: Replace tabs with spaces

Some old code was still using tabs instead of spaces, so replace the leftover
tabs with 8 spaces each.

GNOME/mutter!620
4 jobs for !620 with compositor-destruction-winlist in 7 minutes and 53 seconds
latest detached
Status Job ID Name Coverage
  Review
passed #325193
check-commit-log

00:00:29

 
  Build
passed #325194
build-mutter

00:02:28

 
  Test
passed #325196
can-build-gnome-shell

00:01:43

passed #325195
test-mutter

00:04:54