Add a new "Item" entity, which pulls from the previously-existent
ItemType raw, and add a "PickUp" command which takes the (currently
*only*) item off the ground and puts it into the inventory.
Implement a concrete "Creature" entity, and place one on the screen at
the game startup for testing.
This revealed a bug with drawing when getting the maximum entity
position, but that appears to be fixed now (yay)