Use OffscreenCanvas for main draw loop composition

[2xAA]

Render speeds are good in modV at the moment, but could be better still.

Using OffscreenCanvas to handle the draw loop should allow all drawing features to run freely and not block each other (in theory?).

Though OffscreenCanvas isn't shipped in Chrome yet, it is behind the Experimental Web Platform Features flag (chrome://flags/#enable-experimental-web-platform-features) which can be enabled by the user in the browser or added to the NWJS build.

This is related to #35.

[2xAA]

More extensive code rewrites to implement this have to happen.
At this point I'm thinking split this section of modV out to a separate graph/node-like project, probably called webgl-compositor and let modV use this for its renderer.