EMI: Draw background layers at intervals

Previously, we split the background layers into everything
before SO 15 and after, and drew like that. However, this caused
the pink ship in shi.setb to not be drawn.

This patch draws the layers of the background at intervals of ten.
So if a set has six layers, layer 5 is the background and 4,3,2,1,0
are rendered at sortorder 40,30,20,10,0.