Reuse same tween for monster shaking

Reuse same tween for monster shaking

Now one tween is created and reused to shake a monster when it is
damaged. As a side-effect, the amplitude of shaking is now constant
while it was random in the previous commit.

I decided to use one tween for a monster to avoid creating a memory
leak by leaving unused tweens in memory.

	modified:   ld40.js