Do not run image processing in event loop

It seems that most if not all image processing for layers occurs as blocking code in [the event loop](https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop). This is bad since it prevents any drawing updates or user IO to occur for the duration of that blocking process. For larger images, this is very significant. At the very least image processing should occur as asynchronous code, if not in a separate web worker altogether.