Add ability to select pixels from anywhere onscreen for color picker

[esimkowitz]

Could potentially use something like https://github.com/rustdesk/rustdesk/tree/master/libs/scrap if I can get it to support WASM targets. That would involve something like the MediaDevices API.

It would also require being able to track the cursor outside of the bounds of the window, though I'm going to make that a separate issue as the rest of the color picker/any other sliders I add would benefit from this.

[esimkowitz]

i believe this will depend on #127