Move the arena to the middle of the level.

There was some glitches due to the zoom when the camera touched the border of
the level.
Sets the minimum size of the camera in the arena.

The minimum size is 1280x720. If the camera goes smaller we have an annoying
zoom effect when the players are close.
ptb::demo_level_loader sets the default exit name.

No exit name was set for the loaded level, thus no player was created.
Fix the depth position of some sprites in the forest 1.