Obstacles placed on the edge of the map are rendered over a wall sprite

[joshlory]

Sample repro: https://studio.code.org/s/coursea/stage/6/puzzle/2

Map:

[[0,0,0,0,0,0,0,4],[0,1,1,1,0,1,1,0],[0,1,1,1,1,1,1,0],[0,1,3,1,1,2,1,0],[0,4,1,1,1,1,1,0],[0,1,1,0,1,1,4,0],[0,0,0,0,0,0,0,0],[0,0,4,0,0,0,0,0]]