Fix OW4_DNG4_L5 mid boss, add exit and fix OW2_DGN3 sniper towers

[lgaribaldi]

• OW4_DNG4_L5 columns take too much space and sometimes the bosses ball attack bounces too much between a column and the wall;
• OW4_DNG4_L5 has no exit, and boss rewards ring instead of amulet;
• OW2_DGN3 random white tiles on the floor;
• OW2_DGN3_L3 Archers in towers can never hit the player;
• OW2_DGN3_L4 Archers locked in cells don't do anything.