BUG: fixed test TestPShadowMapPass. The cause of this bug is that all process did not share the same
bounding box for visible objects, because some are visible on proc 0 and not on proc 1.
Compositing the shadow maps had no meaning because they did not share the znear/zfar.
Now, the global bounding box of all props in the scene is used instead.
This looses some precision on the shadow maps, but it is more robust.