LightMap is now cleared when game becomes inactive. Related to issue #5

[This little bug](https://github.com/stefanweck/dungeongeneration/issues/5) was related to the LightMap not being cleared between maps.

Now the LightMap only updates when the game is active. Also added a boolean (tilesLit) to prevent the clear function being called repeatedly if the game is ever inactive for a while (in case of a "game over" screen, menu, etc, in the future.)