Vehicle beds.

Using a seat, vehicle beds can now be constructed.

As comfortable to sleep on as regular vehicle seats,
but does not prompt the player to board the vehicle when walked upon.
Made opaque vehicle doors actually opaque.
Merge branch 'master' into vehicle-parts
Fixed a minor visual glitch with the semi-truck.
Added internal doors for vehicles.

Same materials as a standard door, but:

Is roofed,
cannot be reinforced,
weaker,
opaque.

Mostly exists for the benefit of the sleeper cab in the Semi-truck.
Merge branch 'vehicle-parts'