Per Pixel Destruction for People Playground

Note: only items with a PPD prefix are destructible.

Adds several per-pixel destructible objects. This addon is in a very early phase and many things aren't yet supported.
Major issues are:
- Cannot save/load
- It may or may not completely break your game at random points in time
- Attached wires will disappear
- Does not react to explosions

If you use any of these objects in a contraption, it'll just not work. So don't do that, it's not ready.