Blender Git Loki

Cleanup: Animation, unify FCurve extrapolation

Previously there were two functions for FCurve extrapolation, one for
before the first keyframe, and the other for after the last. After the
previous cleanup made the variable names consistent, it was clear that
the code was almost identical. The biggest difference was in the sign of
many of the calculations, which was cancelled out by swapping `B-A` to
`A-B`. This showed that the computations are actually the same, and the
only remaining difference was which neighbouring handle to use in case
of B�zier curves.

No functional changes.

# Conflicts:
# source/blender/blenkernel/intern/fcurve.c