Fix: monsters no longer flip back and forth

Monsters sometimes shook from side to side: when walking almost straight up or down the screen, the sprite flipped left and right several times a second. This came from small variations in the connection to the server.

Now a monster only turns to a side when its direction of movement is clear, and that direction is smoothed for a moment before it changes. Movement and combat are unchanged: only the way the game draws where the monster is facing changed.

What changed

• Monsters no longer keep flipping from side to side while walking.
• The side a monster faces only changes when it really changes direction.