The goal is to make the evaluation of repeat zones more efficient by making
use of the lazy-function evaluation system. Performance is improved in two ways:
* Unnecessary nodes are not evaluated anymore. E.g. if a repeat zone outputs two
geometries but only one of those is actually used, the other one will not be
computed anymore.
* Support evaluating different iteration indices at the same time on different threads.
It is possible that some uses of repeat zones become slower with this refactor,
especially when each iteration does very little work and there are a lot of iterations.
The old implementation was not optimized for this use case either but now there
is a bit more overhead constant overhead per iteration than before.
On the bright side, this change can result in some very significant speedups when
some computations can be skipped. See #112421 for some example setups.
There is one todo comment for adding back-links for socket usages. Properly linking
those up can result in better (shorter) life-times for anonymous attributes. Even without
that, performance is already better than before.
The implementation reuses the existing lazy-function graph system for the repeat
zone, by building a dynamically sized graph based on the number of iterations.
Building a lazy-function graph makes it possible to use the lazyness and multi-threading
features of the lazy-function graph executor. This is much easier than reimplementing
this behavior for repeat zones specifically (hence there was only single-threaded eager
execution before).
Pull Request: https://projects.blender.org/blender/blender/pulls/112421
dacb768cb6