New vehicle part (underbody wide wheel)

Seeing as nobody wants a campervan with huge holes in the roof, I've
made a new vehicle part: underbody wheels.  For an extra point of
required skill, you can install wheels UNDER your vehicle instead of on
the sides.  This allows you to install interior parts (like roofs) on
the same tile as your wheels, so you don't have big holes in your
ceiling if you want to mount wheels somewhere other than the edges of
your vehicle.  This started as just an addition to get the RV working,
but I think this will actually be a significant addition to vehicle
crafting as a whole.  Now you can make larger vehicles (like moving
houses!) and be able to have wheels in the middle without screwing
things up.  This only adds the wide wheel (underbody) for now; I made it
as a proof of concept to see if this would work.  I'll make the rest of
the wheels tomorrow.

Note - For some reason, this has changed the size of the wheels
displayed in game.  I gave it same wheel width, only changed damage (to
match frame damage, which is essentially what this is).  Not sure what's
causing this, anyone have any ideas?  Also, in a test build the size
seems to change based on multiple wheels next to each other, wierd.