Okay, did a bunch of stuff. Getting ready to create a merge request, but not quite there yet.

* MF_AMBUSH is now MF2_AMBUSH, because it's something you turn on in a map editor, not with a SOC definition.
* Where MF_AMBUSH was is now MF_PAPER.
* MF_PAPER accesses all the stuff I did previously in this branch...
* ...as well as turn on paper-thin collision detection between mobjs, which I've gotten working but isn't perfect but it's still good enough for non-solid objects!!