Refactor Canvas Layers API to use OffscreenCanvas

[joneubank]

This is an experiment to determine if there is significant benefit to this change.

OffscreenCanvas is available to work with bitmap image generation with lower overhead, or potentially in a worker thread. The layer api built onto the Canvas is a nice idea for creating and managing layers using the built in html APIs, but it is very slow. It may get significant performance gains from using offscreen canvases (since we never render these layers, they are just kept in memory for stamping onto the one rendered canvas).