ReturnDestination to main menu fix

[GeorgeFeldy]

Fixed cache.DrawMenu still being called when exiting a subworld to the main menu (with ReturnDestination being int.MinValue) Once the subworld had finished saving and unloading, the cache Subworld still had a value, so the DrawMenu method was still being drawn.

Simply setting the cache value to null once the saving was complete fixed it.

[GeorgeFeldy]

I discovered this while toying around with the possibility of a "Save & Exit" directly from the subworld - this should also include the player rejoining the exact same subworld upon joining a world. This is because in our mod (Macrocosm), we want the players to only be able to go back the same way they came.

Other related things to consider:

• Displaying the usual "Save & Exit" in the settings menu, instead of "Return", when the ReturnDestinationis set to the main menu.
• Storing the last subworld ID of a particular player, and automatically joining that said subworld (alternatively, this should be possible on each Mod's side, with MovePlayerToSubworld, but it would be nice to have a standard way to do that)