Significantly improve the efficiency of how we paint images when full-page scaling, by doing scaling directly with imload.

This avoids a lot of expensive operations (a drawPixmap on a scaled QPainter isn't pretty) and lets us take advantage of imload's internal caching infrastructure.

Unfortunately, this doesn't help with background images; those also need a lot of thought on how to fully take advantage of imload, including unifying of cache and pre-tile management, etc.