Blender Git Loki

Cleanup: Animation, split FCurve extrapolation into separate functions

The `fcurve_eval_keyframes` consists of three parts:
- Before the first keyframe
- After the last keyframe
- Between the keyframes

This commit splits the first two parts into separate functions. This is
the first of a series of refactors, which will be committed into smaller
parts so that each is easier to follow & validate.

No functional changes.