With this commit, the distribute points on faces node runs only once for
every unique mesh in its input. That means if there are 100 instances of
the same mesh, it will only run once.

This basically reverts rB84a4f2ae68d408301. The optimization there
didn't end up being worth it in the end, since it complicates code quite
a lot. It's also incompatible with this method of dealing with instances,
and it breaks field evaluation for instances, where we would have to
make sure to handle each instance transform properly otherwise,
evaluating the field separately for every instance.