[canvaskit] support globalAlpha and dashed strokes

This involves a refactor of how we deal with paint.
Now, we have a source-of-truth paint that is copied
and modified for each of fill/stroke/shadow.

Adds preliminary support for blend modes.

Most of what remains is text, images, and gradients.

Bug: skia:
Change-Id: I41806adeb7de4faa6c98a580c4f1de4e4a34a37d
Reviewed-on: https://skia-review.googlesource.com/c/173223
Reviewed-by: Kevin Lubick <kjlubick@google.com>
6 files changed