[canvaskit] Save a copy on CPU backend by using RasterDirect

Draw directly into an unpremul buffer (which is what Canvas's
get/putImageData is) and it speeds up Skottie animation by
about 3x.

Bug: skia:
Change-Id: Ie131276e3a675e0e5a35190da93f34d7469624e4
Reviewed-on: https://skia-review.googlesource.com/c/173644
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed