Support OffscreenCanvas

Spector does not detect canvas controlled by a WebWorker. When explicitly capturing canvas, it produces error.

    const offscreenCanvas = (renderCanvas as any).transferControlToOffscreen();
    const spector = new SPECTOR.Spector();
    spector.displayUI();
    spector.captureCanvas(offscreenCanvas);

This produces error:

TypeError: e.getAttribute is not a function

Or:

No webgl context available on the chosen canvas.

If the DOM canvas is passed to captureCanvas().

I tried using it inside WebWorker to capture data in JSON, send it to DOM thread and display it there. But it throws error when imported:

ReferenceError: window is not defined

[cdata]

@sebavan I made an attempt in #129 - LMKWYT!

[sebavan]

Yup makes much more sense to me 😃

If you need it pretty soon, feel free to make a PR, I am currently focusing my spare time (pretty tight) to not drop Firefox support due to review issues.