[canvaskit] adding drawAtlas to canvas API

Also re-works colors in the JS part to be unsigned,
so there's less chance for oopsies when passing them
to the C++ side.

Change-Id: I3f56d22e873e1407c402606444c50b76dd34ffd4
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204543
Reviewed-by: Mike Reed <reed@google.com>