Support OffscreenCanvas

[nathanbabcock]

• Handle OffscreenCanvas in browser loadImage function
• Update ImageLike type to include OffscreenCanvas
• Add unit test covering recognize from an OffscreenCanvas
• If OffscreenCanvas is not supported in the browser, the relevant branch in loadImage will be safely skipped over without any errors

Other references:

• https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas
• https://caniuse.com/offscreencanvas
• https://caniuse.com/mdn-api_offscreencanvas_converttoblob
• See also: Improve loadImage performance by between 20% and 100% #726

[Balearica]

This implementation breaks Tesseract.js for users who do not have a browser that supports OffscreenCanvas, which includes all non-Chromium users who have not updated their browser recently. I'm not opposed to adding handling for OffscreenCanvas inputs, but that aspect needs to be fixed.

[nathanbabcock]

You're quite right of course.

The check I was using -- if(OffscreenCanvas) -- was inadequate and it still produced a ReferenceError in older browsers. I've replaced this with if (typeof OffscreenCanvas !== 'undefined'). To verify this, I downgraded to an older version of Firefox, v72.0.2 from January 2020, which does not natively support OffscreenCanvas. Before this change, I saw an error when running the Tesseract.js demo app -- after the commit, it works smoothly.

Thanks for your time and let me know if you notice any other problems.

[Balearica]

Thanks for editing, as well as checking using the old version of the browser. The new implementation looks good, I will merge.