Loading a save from a previously unloaded map leaves unloaded images

[darkf]

Ideally it should load all relevant images like loadMap does (either we should split that off from loadMap or we should use loadMap. Probably the former, since we don't need loadMap's logic.)

[darkf]

I changed the renderer to try to lazy load missing tile images, but we could do this up front too.

Or maybe this is the better method, since we can drop the *.images.json metadata.