main.lua, etc. - use a sorted list of mobs

The mobs are characters to animate.  They're sorted by screen depth,
so that nearer ones render over farther ones.

arena.lua

@@ -52,6 +52,8 @@ return function(size, discs, topCol, rightCol, leftCol)
    end
 
    return {
+      getDepth = function() return -1; end,
+      
       getWidth = function()
 	 return size*width*2
       end,