poor man's drop prevention

ttrs.py

@@ -338,6 +338,7 @@ class TtrsGame(threading.Thread):
         TICK_TIME = 0.1
         FALL_INTERVAL = 5
         MOVE_INTERVAL = 1
+        DROP_BARRIER = 4
         ticks = 0
         lastfall = 0
         lastmove = 0
@@ -398,7 +399,8 @@ class TtrsGame(threading.Thread):
                             to_add.add(newb)
                     self.playground.blocks.update(to_add)
 
-            if ticks - lastfall >= FALL_INTERVAL or evt == PlayerEvent.DROP:
+            drop = evt == PlayerEvent.DROP and mino.pos.y > DROP_BARRIER
+            if ticks - lastfall >= FALL_INTERVAL or drop:
                 lastfall = ticks
                 try:
                     mino.move(Point(0, 1))