[canvaskit] Add sundry APIs and tests

To the reviewer: I've tried to make it so each PS adds one new API.

Change-Id: I81fc85c7a93a19ce4fd725a125e138d35471e693
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/237155
Reviewed-by: Mike Reed <reed@google.com>
7 files changed