Set the size for Icons.Loader ImageLayer's.

https://github.com/threerings/tripleplay/issues/47

The suggested fix would work but would also make things more confusing. The setSize call in
Preloader would have no effect because ImageLayer takes it's size from the image by default.

So instead, change the class hierarchy to reflect what Preloaded and Loader really have in common,
then add the setSize call in Loader, where it's really needed.
Simplified.

I think this was being overthought. ImageLayer is capable of being told its
dimensions and rendering nothing "at that size" until the image is ready. No
need to bring in a GroupLayer.