consider supporting incremental rendering for non-zoom again

[thestinger]

Rendering is currently done in an off-screen canvas and the page isn't updated until the rendering is completely finished. Ideally, it could render to a visible canvas to display the progress, but this proved to be incredibly slow compared to offscreen rendering. Perhaps it could regularly copy over the canvas as a way to work around that. It's possible that pdf.js will improve this at some point so that working around it won't be required anymore and it can be reverted to how it used to be rather than using the offscreen canvas.