Use offscreen canvas if the global scope is worker

[hajimehoshi]

https://developers.google.com/web/updates/2018/08/offscreen-canvas https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas

If Ebiten can work on a worker, multiple Ebiten instances can work in theory.

[hajimehoshi]

We need to have API to pass an (offscreen) canvas. maybe.

[hajimehoshi]

https://caniuse.com/offscreencanvas

Only Chrome (and Edge) support this.

[superloach]

OffscreenCanvas landed in Firefox 105, still only at 73% usage though. Although it's not the worst to expect users to use a fairly new browser for a feature like this.

[hajimehoshi]

https://caniuse.com/offscreencanvas Now most modern browsers support this

EDIT: The latest Safari doesn't support it yet.

[hajimehoshi]

In workers, inputting would not work, but probably this is fine...