Eliminate Game_Map and Game_Battle global state

[mateofio]

These are 2 singletons which store a lot of random state in unmanaged global variables.

Instead of having these 2 namespaces full of junk, I suggest we fold all of this into Scene_Map and Scene_Battle. This will ensure when these scenes are recycled, all the globals get reset properly. It also simplifes our code to have 1 place to find these things.

We'll still need globally scoped access to map and battle data. So we'll need a fast way to access these scenes.

One suggestion I was thinking of.

• For each scene type, have a global std::unique_ptr<Scene_Type> Scene_Type::instance variable. These means we can't have multiple scenes of the same type on the stack, but we don't ever need that anway.
• Wrap this up with global acessors:
  • GetScene() which returns a reference and fails if the scene is not active. This is for code which should never run when the scene is not created.
  • GetScenePtr() which returns a pointer and can be used when scene is active or not.
• Modify the scene stack to just store a stack of raw pointers, pointing to these.

We could potentially still have these Game_Map and Game_Battle namespaces as accessors for this stuff, but the data would all be stored in the scene.