This commit makes the bounding box node work on each unique geometry
(including instances) individually instead of making one large bounding
box for everything. This makes the node much faster, and is often the
desired result anyway. For the old behavior, a realize instances node
can be used in front of this node (versioning adds it automatically).

The min and max outputs now only output the values from the realized
geometry.

I went back and forth about what should happen here a few times.
At first I thought only the top level of instances should be considered
and that nested instances should be included, but I think it's simpler
in the end if we stick to what every other node is doing and treat
nested instances as separate geometry sets as well.