Fix scenario loading to game state

[xsebek]

See the discussion in #515:

Changes to the entity map are permanent. For example, to reproduce:

1. Start Swarm in cheat mode
2. Load the new testing scenario containing Tardis ("Install one device")
3. Quit to the menu
4. Start a creative mode game
5. Execute create "Tardis": it should say "I've never heard of a Tardis", but instead, it works. That's because the Tardis was added to the entityMap when loading the test scenario, and it was never removed.

We will have the same problem with recipes.

The problem is that scenarioToGameState modifies the existing GameState instead of creating a brand new one from scratch. I am actually not sure right now whether there are good reasons for this. But I think the "right" solution, assuming it is not too difficult for some reason I can't think of right now, is to create a new GameState from scratch every time we load a new scenario. But if we don't want to actually reread the entities and recipes from disk every time, we will have to save copies of the "pristine" versions somewhere, which is what my comment do we need to keep an old entity map around??? was supposed to mean.

Solution

Store the original game state (or at least entity map and recipes) and update it when loading the scenario.