Sprite enhancements

[ludwigschubert]

Things I'd like; maybe together with the asset loader:

pre-loading hints

I'd like to be able to tell the asset loader to go ahead and request my asset. For example on hovering over an example chooser.

debouncing

We added the grey loading screen to invalidate the current sprite so we don't show outdated images while the new ones are loading. This works great, but would be even better if it only set the grey loading background after a short delay, such as 100ms to prevent flashing.