CanvasKit: implement client-supplied dirty rect support for software surface.

Surface clients can now specify a dirty rect that will constrain HTML canvas
repaints to a particular region.

Bug: NONE
Change-Id: Ia77d6e864c061297bfc82cfa09030523052ee0e1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293056
Reviewed-by: Kevin Lubick <kjlubick@google.com>
4 files changed