object-group draw-order

[Zulu-Inuoe]

Currently, a cl-tiled:object-group has a list of objects, and a draw-order property. The objects are in whatever order we happened to encounter them, but it might be useful to sort them as part of loading a map according to draw-order, so that renderers need only iterate through the list for correct render order.

The same idea applies to the map's render-order, when determining the tiles in each layer.