[canvaskit] Add docs/types for Paint and Filters/Effects

This also includes matrices (which I'm reminded I want to test with
TypedArrays and make optional).

Bug: skia:10717
Change-Id: I0f6565a46b766c1f81c28dfc1229d403bff32e69
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321118
Reviewed-by: Nathaniel Nifong <nifong@google.com>
4 files changed