TiledMap uses ObjectMap, override equals/hashcode for GridPoint.

I am writing this openstreetmap extension for Libgdx. However I ran into memory problems when using the TiledMap-API. When creating a new layer, the layer immediately allocates Cell[width][height]. For potentially large maps this is problematic (gives JVM out of memory exception). This solution uses ObjectMap instead of allocating the arrays. However, I also had to override equals/hashcode for GridPoint (which is used as the key).