API: Use LineDef (and not Seg) with P_SetPolyobjCallback()

The original game would use the seg angles for the purpose of testing
collisions with mobjs. This rather defied the design of the idtech 1
engine which otherwise used linedefs everywhere else for this purpose.
This exposing of a polyobj's segs to the playsim is unnecessary.

Polyobjs now use linedef angles for collision purposes.

doomsday/engine/api/doomsday.h

@@ -516,15 +516,16 @@ void            P_SpawnDamageParticleGen(struct mobj_s* mo, struct mobj_s* infli
  */
 ///@{
 
-    // Play: Polyobjs.
-    boolean         P_PolyobjMoveXY(struct polyobj_s* po, float x, float y);
-    boolean         P_PolyobjRotate(struct polyobj_s* po, angle_t angle);
-    void            P_PolyobjLink(struct polyobj_s* po);
-    void            P_PolyobjUnLink(struct polyobj_s* po);
-
-    struct polyobj_s* P_PolyobjByID(uint id);
-    struct polyobj_s* P_PolyobjByTag(int tag);
-    void            P_SetPolyobjCallback(void (*func)(struct mobj_s*, void*, void*));
+// Play: Polyobjs.
+boolean P_PolyobjMoveXY(struct polyobj_s* po, float x, float y);
+boolean P_PolyobjRotate(struct polyobj_s* po, angle_t angle);
+void P_PolyobjLink(struct polyobj_s* po);
+void P_PolyobjUnLink(struct polyobj_s* po);
+
+struct polyobj_s* P_PolyobjByID(uint id);
+struct polyobj_s* P_PolyobjByTag(int tag);
+void P_SetPolyobjCallback(void (*func)(struct mobj_s*, void*, void*));
+
 ///@}
 
 /// @addtogroup material

doomsday/engine/portable/src/p_polyob.c

@@ -31,7 +31,7 @@ static void rotatePoint(int an, float* x, float* y, float startSpotX, float star
 static boolean checkMobjBlocking(seg_t* seg, polyobj_t* po);
 
 // Called when the polyobj hits a mobj.
-static void (*po_callback) (mobj_t* mobj, void* seg, void* po);
+static void (*po_callback) (mobj_t* mobj, void* lineDef, void* polyobj);
 
 polyobj_t** polyObjs; // List of all poly-objects in the map.
 uint numPolyObjs;
@@ -473,7 +473,7 @@ int PTR_checkMobjBlocking(mobj_t* mo, void* data)
             if(P_BoxOnLineSide(&moBox, params->lineDef) == -1)
             {
                 if(po_callback)
-                    po_callback(mo, params->seg, params->polyobj);
+                    po_callback(mo, params->lineDef, params->polyobj);
 
                 params->blocked = true;
             }