Script sprite rebuild unloads sprites that may still be referenced in save states

[msg555]

Right now there is a single global SpriteManager that holds

• An array of all loaded sprite objects
• A mapping of names to sprite sets

When a map with scripts is reloaded it will trigger the "scripts" sprite set to be rebuilt which will unload all sprites it contains. If there is a save state that has script sprites then it will likely fail once resumed when it attempts to render or access metadata about a sprite.

[msg555]

I'm removing this from the milestons and just putting it in the backlog as this is a big project with minor payoff