Sector_Attach3dMidtex(line_id, tag, do_ceiling)
This is a static special, activated at map startup.
Attaches 3DMidTex lines to the vertical movement of the front sector's floor or (if do_ceiling is 1) ceiling. Normally it will seek lines tagged line_id, but if tag is non-zero, it can restrict search to lines belonging to tagged sectors. If line_id is zero while tag is nonzero, it will find all 3dmidtex lines belonging to sectors with tag, regardless of line tags.