fix: Mazeweb.activity sometimes having player without face.

The Unexpected behaviour mentioned in issue #1536 occurs randomly it's not related to restarting the activity or anything.
The behaviour is due to calling runLevel function several times without clearing the dirtyCells array/state.
if we call runLevel again and dirtyCells still have some cells left to draw from previous level, it'll try to draw it in another level.
I also reproduced the unexpected behaviour by setting maze.goalPoint to same as maze.startPoint (which was in actual code happening randomly) and then with setTimeout changing the goalPoint again.

llaske / sugarizer