add vd/mv onedir hotswap

[zeffii]

this adds a hotswap macro, it can be seen as a template.
i opt to make it one-directional for now, because it's usually the way the conversion makes most sense.

future improvements:

• it could also pass a new material basecolor. to match the face color found in vdmk4.
• make sure it respects being framed, which it does not do at the moment.