Implement new 'OffscreenCanvas' to enable WebGL processing in a webworker

[GordoRank]

It should be extremely trivial to make use of the new "OffscreenCanvas" feature to allow the webGL resizing to happen in a webworker.

Combining WebGL and webworkers in this manner would enable maximum speed, flexibility and responsiveness. It can currently tested by turning on the "about:config" flag "gfx.offscreencanvas.enabled" in Firefox or Firefox Nightly.

Further information can be found here:

https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas

With a lot more detail here:

https://hacks.mozilla.org/2016/01/webgl-off-the-main-thread/

Firefox has a completed implementation, behind an experimental flag, Chrome has it in development and MS is soon to start.

[puzrin]

Closing until supported in major browsers and util webgl works in pica with production quality.