Improve caustics computation performances

[martinRenou]

As suggested by someone on Twitter, we could use hierarchical env maps in order to speed up the caustics computation. This would reduce the search space for the intersection and might increase the precision while increasing the performances.

We should also render the envmaps from the POV of the refracted rays (taking the assumption that the water is completely planar) so that the performances don't depend on the light incidence anymore.