[skottie] Allow clients to override image scaling mode

Add a new FrameData field, so clients can specify a different strategy
than the default kCenter_ScaleToFit.

Bug: skia:13966
Change-Id: I4fb6170de8efce4546b45c9a9d023113f71938d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/612676
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
3 files changed