Inverted plane displacement

If you don't know of the new linedef types 66, 67, 68 or how they work already, see !61's description.

This is basically a quickly-thrown together branch to support inverse plane displacement; just simply turn on Not Climbable on any of the plane displacement linedef types to enable this for any sectors tagged by them. This causes moving the control sector to move all tagged sector planes in the reverse direction.

I've provided srb2win-inverseplanedisp.exe and plane-disp-test2.wad in my folder on the FTP to test this branch.

See merge request !79