Changed TGPlayer object to start in the middle of the screen in preparation for having them permanently there while the scene moves around them.
The player is now fixed to the center of the screen; the scene moves around the player and will stop when the edge of the scene hits the edge of the window. Scenes will have to be drawn such that there is scenery (of a fixed distance on each side) outside of the active area where the player can walk. If, say, we want the player to be able to enter a door at the edge of the screen, the door will then have to be in the middle where the player can reach it.