Performance: investigate ctx.willReadFrequently, offscreenCanvas, worker

[joedf]

just add ctx.willReadFrequently = true; in ComputeProbeValue_gs() seem to produce a minor speed improvement... but maybe it's now CPU-bound? maybe a context should be passed instead of an image/canvas if possible?

https://caniuse.com/?search=willReadFrequently

[joedf]

• https://developer.chrome.com/blog/offscreen-canvas/
• https://mdn.github.io/dom-examples/web-workers/offscreen-canvas-worker/
• https://konvajs.org/docs/sandbox/Web_Worker.html
• https://www.macarthur.me/posts/animate-canvas-in-a-worker

[joedf]

Note to self: tested again today, the offscreenCanvas commit is the largest performance boost; a 137x zoom image went from ~50ms to ~15ms per row in the SEM live view.

1. Commit where offscreenCanvas is being used: 7737a73
2. Commit just before it: 8ddaf9f
3. newest commit presently: 6577079

1 & 3 performed the same, 2 was slowed and eventually crashes the tab when switching away and back.