fixed incorrect drawing of players which are moving next to each others

fixed incorrect drawing of players which are moving next to each others

Before, players which were moving next to each others without a free
tile between them were drawn incorrectly (with (usually black)
background tile graphics partially drawn over one of the players).

This problem is now fixed by drawing all player tiles layer by layer
(first drawing the background tiles for all players, then drawing the
players, then drawing elements in front of the players etc.).