Custom Walls could have a customizable render order

[Morxemplum]

I've already lined out a basic attribute for the render order in an upcoming PR but seeing how this is turning out more complicated than I am expecting, I've decided to put a pin in that feature. I think it is something that should be addressed later. I think it would be really nice to explicitly determine when the wall should be rendered rather than based on creation time.

Besides, it would also be nice to control whether the wall gets rendered over and under certain elements (like the polygon and player, along with the basic walls). I think this was an initial critique with the custom walls system, and it's good that we can finally address these issues and make the custom wall system more powerful.