[skottie] Time start, time stretch

Pre-compositions (only) can have their timelines adjusted via bias and
scale.

TBR=

Change-Id: I519fa1d7cf210f7f152dcabcbe004119a2cf08d9
Reviewed-on: https://skia-review.googlesource.com/101460
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed