[skottie] Frame-index seek API (seekFrame)

Add a new seek method which takes a frame index. This provides more accurate
control when rendering of exact frames is needed.

Also surface the animation frame rate, and convert skottie_tool to use this
new mechanism (now defaulting to native animation frame rate).

Change-Id: Id870629e1747a9f70cbf4d3e770366df1299a519
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249799
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
3 files changed