Vehicle door curtains

This enables curtain installation on vehicle doors. They function the same way as curtains on windshields, and the interaction happens similarly to domestic window interaction (ie, open curtains first, then open door). 

One caveat is that if you try to open a door with a closed curtain from outside, it'll open both the door and the curtain. This may be kind of odd, but I figured it'd be simpler from both interaction and implementation perspectives—alternatives would require more rules for figuring out tile opaqueness, and possibly extra menus when opening things.