clutter/paint-volume: Use graphene for computing union of paint volumes

Apparently clutter_paint_volume_union() has problems building the union
of two paint volumes in eye coordinates, that's probably because of the
negative coordinates that come into play there.

Circumvent that by making even more use of Graphene and letting it take
care of computing the union. We do that by creating two graphene_box_t's
from the axis-aligned paint volumes and intersecting those boxes, then
setting our vertices to the new min and max points of the resulting box.

!1507
6 jobs for !1507 with clutter-paint-vol-fix-union in 6 minutes and 26 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #943678
check-commit-log

00:00:33

 
  Build
passed #943679
build-mutter

00:02:05

passed #943681
build-without-native-backend-and-wayland

00:01:44

passed #943680
build-without-opengl-and-glx

00:01:29

 
  Test
passed #943683
can-build-gnome-shell

00:01:38

passed #943682
test-mutter

00:04:20