Merge branch 'plane-displacement' into 'master'

Plane displacement

Basic support for plane displacement, no netplay support as of writing but this is what Nev3r wanted moreorless at least.

Linedef types 66-68 are the plane displacement specials; 66 = move floor only, 67 = move ceiling only, 68 = both

Front sector = control sector
Tag = tag of target sectors to be moved whenever the control sector floor moves
Linedef length = movement factor relative to control sector movement, 256 = 1:1 with control sector. 128 is half as much, 512 is twice as much, etc.

Whenever the control sector floor moves (not ceiling, never ceiling), the selected planes of the tagged sectors all move (mind, they have a tic delay due to how it all works, it's not perfect really). No support for reverse speeds yet sorry.

EDIT: oh, btw, test exe and test map are included in my folder on the FTP (srb2win-plane-displacement.exe and plane-disp-test.wad, respectively)

See merge request !61