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.

hopper.lua

@@ -162,6 +162,8 @@ return function(arena, draw, update)
    local se, sw = 0, 0
    local jumpPath = newJumpPath(arena.getColWidth(), arena.getRowHeight())
    return {
+      getDepth = function() return sw+se; end,
+      
       jump = function(dir)
 	 if jumpTime ~= nil then
 	    return -- already jumping