[skotty] Add cubic Bezier lerp stubs

... and refactor some of the keyframe parsing.

TBR=
Change-Id: If45922eab36412908036401cee693202f5c3e281
Reviewed-on: https://skia-review.googlesource.com/91100
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed