Add support for main thread OffscreenCanvas

[cdata]

This change proposes support for OffscreenCanvas on the main thread.

In general it seems like adding support for OffscreenCanvas is fairly trivial, the main requirement was to ensure that the correct constructor is referenced, and secondarily that no HTMLElement methods are used when dealing with an OffscreenCanvas.

I'm not very familiar with the code base overall, and I couldn't find tests, so it is difficult to tell if I patched everything correctly. But, I was able to record frames from an OffscreenCanvas in Chrome.

Fixes #90

[sebavan]

This is already Christmas !!!

Thanks a lot, I do not have unit tests per se only test cases in the samples folder that I manually play, I ll just add one there for offscreen and publish again over the week end, thanks a lot for the PR.