Add vehicle part curtains.

Curtains can be installed on vehicles using up a sheet, they can only be
opened or closed from inside the vehicle.
Force the player to be inside to open the curtain
Mount curtains on windshields only

And apply the right color for closed curtains.
Remove curtain when windshield is destroyed
Fix small typo
Merge pull request #4358 from axujen/veh_curtains

Merge PR#4358 - Vehicle curtains.