Blender Git Loki

FCurve RNA API: add funcs to convert to samples/to keyframes.

So far, we had an operator to 'bake' keyframe curves into samples, but no
way to make the fcurve editable again (i.e. to convert it back into a keyframes one).

Needed to fix mocap addon (see T43259).

Also, fixed a glitch in `fcurve_store_samples()`, since given end frame is included in range,
it is valid to give same start and end frame (in case you want a single point in samples,
not much practical cases, but...).