ClutterStage defines the 8 vertices of a frustum:
4 ----------------------------- 5
| \ / |
| \ / |
| 0 --------------------- 1 |
| | | |
| | | |
| 3 --------------------- 2 |
| / \ |
| / \ |
7 ----------------------------- 6
Then, it uses triplets of vertices to create each clipping plane.
It only sets up 4 planes (it doesn't clip based on depth), defined
by the following vertices:
* 0 - 4 - 5
* 1 - 5 - 6
* 2 - 6 - 7
* 0 - 7 - 4
The first 3 triplets are selected using the for-loop. However, the
last triplet is different, and is done out of the loop. It could
have been made simpler by using the "3 - 7 - 4" triplet.
Simplify the current code by using the suggested triplet, calculated
inside the for-loop.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1489
175851eef7