Better bounding volumes for better near/far plane computation
Right now, we only compute the near and far planes when shadows are enabled and
compute tight scene bounds is too. This stops shadow map space being wasted on empty space below ground.
We have to compute the near and far planes based on primitives as (potentially among other things) the water plane has an undesirable bounding volume, so we can't use bounding volumes. This is accurate, but slow. A faster, but still sufficiently accurate alternative could be implemented.