Refactor to prepare for slotted action support to the evaluation of
quaternion F-Curves.
Since slotted Actions store F-Curves in an array, you cannot iterate over
them any more via the `ListBase` pointer `fcurve.next`. Quaternion
evaluation code has been refactored to work on a span of F-Curves instead
of just getting the first one.
For now, slotted Actions just evaluate their first slot only. A future
commit will add a slot handle parameter to evaluate the correct slot.
7cd3f2aabb