[canvaskit] Create an SkImage from a frame of an SkAnimatedImage

Flutter on the web wants to be able to extract arbitrary frames from
an animated image and pass those into functions like:
drawAtlas, drawImage, drawImageRect

This should allow that to happen w/o having to add lots of variants like
drawAnimatedImage. If this sticks, is drawAnimatedImage still useful?
(maybe it saves a copy?)

Change-Id: I99d7045c5dea61d0a1bd6d335c88e7517f2c4fc2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/263020
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Reed <reed@google.com>
6 files changed