Deno: OffscreenCanvas and CanvasRenderingContext2D

[codehz]

1629 is about WebGL, it will likely take a lot of time to implement.

I think add OffscreenCanvas & CanvasRenderingContext2D interface at first is a good idea for now.
A lot of server-side image process like shields.io can be done in OffscreenCanvas with 2d renderer
(and it should be able to load image from disk / web / memory.)

• Maybe render text will be a problem
  Read from ttf directly? <- so we need a FontFace api with local file support
  Use system font registry?

[tdillon]

I've been eagerly awaiting canvas since I heard about Deno.