The Realize Instances and Join Geometry node can share most of their code.
Until now, both nodes had their own implementations though. This patch
removes the implementation in the Join Geometry node in favor of the more
general Realize Instances implementation.
This removes an optimization for avoiding spline copies. This can be brought
back later. The realize instances code has to be rewritten to support attribute
instances anyway.
Differential Revision: https://developer.blender.org/D13417
506d672524