Blender Git Loki

Animation: Refactor storage usage during fcurve modifier evaluation

Previously, when a fcurve modifier used storage,
many heap allocations were done.
This caused major slowdowns as described in T63656.

Furthermore, the storage usage was a special case only
used by the Cycles modifier. This refactor makes
storage usage the "normal" case.
That reduces the overall complexity.

The storage is stack allocated now.

The framerate on the provided test scene went up from ~5 fps to ~16 fps.

Reviewers: angavrilov

Differential Revision: https://developer.blender.org/D4701