Improve handling of spritesheets

[tredfern]

No legitimate way to deal with spritesheets, which is leading to scenarios where downstream utilities need to know about quads and how to slide the sheet.

This is causing issues when serializing the state store as quad and other userdata can be mixed in. While userdata can be serialized, it's problematic to have this data saved out when it can be rehydrated through the standard mechanisms as long as stateful entities can reference the correct image/quad data.

Ultimately, quad and image data should be abstracted away enough to allow for simplified serialization.