[canvaskit] Return damage rect from ManagedAnimation::seek()

This is useful to avoid redrawing unnecessarily when the animation
doesn't progress.

Bug: skia:9267
Change-Id: Id4184ae8308b8abd959fbfd1768e3e22d1efe0a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229006
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed