clutter/paint-volume: Union paint volumes correctly using Graphene

Since commit d2f8a306 we use Graphene to union paint volumes, it turns out a quite severe issue snuck in during review of that MR though: Unioned paint volumes (so paint volumes of any actors with children) now have negative heights. Once projected to 2d coordinates they luckily are correct again, which is why everything is still working.

The problem is that obvious once looking closer: For the y coordinates of the unioned paint volume we confused the maximum and the minimum points and simply used the wrong coordinates to create the unioned paint volume.