Initial implementation of server-side reconciliation for player
movement.
Clients respect the server as the authoritative source for game state.
In 'process_game_state', the client compares its locally predicted pos
with the state received from the server and snaps to the server's
position if they diverge.
Had to add a defensive check to the server-side Player constructor to
fix a bug where the first connecting client was incorrectly assigned an
ID of 0. This prevented it from sending or recieving any game data.
