[canvaskit] Add gradients and clips

Refactors things inside to support both color
and gradient as _fillStyle/_strokeStyle.

Bug: skia:
Change-Id: I364ceb7d55c41e11161d5577dcd1611a592bbc29
Reviewed-on: https://skia-review.googlesource.com/c/173421
Reviewed-by: Kevin Lubick <kjlubick@google.com>
6 files changed