Offscreen canvas project

Implement OffscreenCanvas API

Background information: The HTML specification defines a <canvas> element that can use a 2d or 3d rendering context. A new specification was recently developed that defines a canvas that can be used without being associated with an in-page canvas element, allowing for more efficient rendering.

Tracking issue: https://github.com/servo/servo/issues/14627 (please ask questions here)

Initial steps:

• email the mozilla.dev.servo mailing list (be sure to subscribe to it first!) introducing your group and asking any necessary questions
• create the OffscreenCanvas and OffscreenCanvasRenderingContext2d interfaces with stub method implementations
  • hide the new interfaces by default by adding a [Pref="dom.offscreen_canvas.enabled"] attribute to each one and add a corresponding preference to resources/prefs.json
• enable the existing automated tests for this feature by adding the offscreen-canvas directory to tests/wpt/include.ini
  • add a __dir__.ini file to tests/wpt/metadata/offscreen-canvas which enables the new preference, then run the tests in that directory and update the expected test results.
• implement the OffscreenCanvas constructor that creates a new canvas
• implement the OffscreenCanvas.getContext ignoring the WebGL requirements, using HTMLCanvasElement.getContext for inspiration

Subsequent steps:

• extract all relevant canvas operations from CanvasRenderingContext2d into an implementation shared with OffscreenCanvasRenderingContext2d
  • create a trait that abstracts away any operation that currently uses self.canvas in the 2d canvas rendering context, since the offscreen canvas rendering context has no associated <canvas> element
• implement the convertToBlob API to allow testing the contents of the canvas
• support offscreen webgl contexts in a similar fashion to the 2d context, by sharing relevant code between OffscreenCanvasRenderingContext2d and WebGLRenderingContext