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.