Add back side wall scrollers & allow using offsets for tagged wall scrollers.

This branch adds linedef types 507 and 508, which scroll the back side textures of their linedef, using either the front or back side texture offsets.

Linedef types 502 to 504 can now also scroll the back side textures of tagged linedefs, by checking the Peg Midtexture (Effect 3) flag, and use texture offsets instead of linedef length/direction by checking the No Midtexture Skew (Effect 2) flag. This allows for scrolling both sides of a linedef, by simply applying two of these wall scrollers.