"Artboards" are top-level Rive containers (similar to AE compositions),
holding the scene graphics and related animations.
Artboard properties:
- name
- width/height (size)
- translation (position)
- origin (anchor point for transforms?)
- (background) color
- clip contents flag
Plumb artboard parsing + background rendering, and hook into viewer.
TBR=
Change-Id: Ib188245ce41a76197cf9e0937689adf8243826d6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295244
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
