Blender Git Loki

Cleanup: Animation, split fcurve_eval_keyframes() into separate functions

There are three main parts of the `fcurve_eval_keyframes()` function:

- before or on the first keyframe
- after or on the last keyframe
- between those points

These are now separated into separate functions. Apart from moving the
code, there are minimal code/cleanup changes. More cleanups to follow.

No functional changes.