[canvaskit] Expand canvas2d API

Made addPath take one more arg to allow for append/expand
(which makes emulating the HTML canvas easier).

Add Gold test for various lineTo/pathTo, etc.

Make CanvasKit.Color() choose a better value for alpha
when omitted (was 0, should be 1).

Add some parsing logic to deal with colors/font sizes.
Fonts are going to be rather complex it seems.

Moves some arc-related logic to the JS side, (although
this should preserve the behavior of CanvasKit.arc() to
behave like the Canvas implementation)

Make Examples and tests to a side-by-side comparison to
HTML canvas where applicable.

Add a Changelog for PathKit.  There was a bug (I thought), but
turns out I was wrong.  The Changelog will be for future
bug fixes.

Bug: skia:
Change-Id: I1bd603fdb518232604b098e24543e3453015b504
Reviewed-on: https://skia-review.googlesource.com/c/170446
Reviewed-by: Kevin Lubick <kjlubick@google.com>
13 files changed