This implements four optimizations in the Set Position node:
* Check whether the position input is the current position and ignore
it if it is. This results in a speedup when only the Offset input is used.
* Use multi-threading when copying to computed values to the
position attribute. All geometry types benefit from this.
* Use devirtualization for the offset and position input. This optimizes
the common case that they are either single values or computed
in the fly in a span.
* Write to `Mesh->mvert` directly instead of creating a temporary span.
This makes setting mesh vertex positions even more efficient.
In my simple benchmark I'm using a White Noise node to offset the
position of 1,000,000 vertices. The speed is `20 ms -> 4.5 ms` in the
multi-threaded case and `32 ms -> 22 ms` in the single-threaded case.
602ecbdf9a