Release 3.1 - prebake caching

Caching of prebaked surfaces

When prebaking the same sprite (i.e. the same sprite_index) multiple times, the OutlineObjectBaked now recognizes this and reuses the surface, so prebaking happens only once per sprite prebaked.

But each prebaked sprite is its own instance and can have its own image_speed, animation_speed, etc. They are all independent but are based on the same surface behind the scenes.

As always,
have fun!

Yours, Gris