This improves performance in cases where the Set Position node is "turned off"
by passing `false` into the selection input.
It's possible that the node still takes some time in this case currently, because
it is destructing the input fields which may reference geometries that need
to be destructed as well. We could potentially change this node (and others)
so that the field inputs are only requested when the selection is not a
constant `false`.
Jacques Lucke
244c87dd68