Visually hide on_roof parts of the vehicle you are currently in

The smartest place to install solar panels is generally on the
interior roof, where they're less likely to be shot at or collided
with.

Unfortunately this also means that every tile of your RV ends up
looking the same.

This PR hides things on the roof of the vehicle you're currently
inside. You can walk around inside, seeing what's inside, as that's
what your character is going to see while they're inside.

Only display of the active vehicle is changed. Other nearby vehicles
display the same way they always have.

A few things (anything not BOARDABLE, notably trunks and empty frames)
are not considered by the existing logic to be "inside" the vehicle
for passenger purposes, and therefore standing in them gives you the
outside view.

Those same parts are the ones you can't step into if you let go of the
controls in a moving vehicle.