Check TRIPWIRE texture at map load

So it can be animated.

src/p_map.c

@@ -1554,8 +1554,7 @@ boolean P_IsLineBlocking(const line_t *ld, const mobj_t *thing)
 
 boolean P_IsLineTripWire(const line_t *ld)
 {
-	return (sides[ld->sidenum[0]].midtexture ==
-			R_TextureNumForName("TRIPWIRE"));
+	return ld->tripwire;
 }
 
 //

src/p_setup.c

@@ -1044,6 +1044,8 @@ static void P_InitializeLinedef(line_t *ld)
 	ld->validcount = 0;
 	ld->polyobj = NULL;
 
+	ld->tripwire = false;
+
 	ld->text = NULL;
 	ld->callcount = 0;
 
@@ -1941,6 +1943,12 @@ static void P_ProcessLinedefsAfterSidedefs(void)
 		ld->frontsector = sides[ld->sidenum[0]].sector; //e6y: Can't be -1 here
 		ld->backsector = ld->sidenum[1] != 0xffff ? sides[ld->sidenum[1]].sector : 0;
 
+		if (sides[ld->sidenum[0]].midtexture ==
+				R_TextureNumForName("TRIPWIRE"))
+		{
+			ld->tripwire = true;
+		}
+
 		switch (ld->special)
 		{
 		// Compile linedef 'text' from both sidedefs 'text' for appropriate specials.

src/r_defs.h

@@ -420,6 +420,8 @@ typedef struct line_s
 	size_t validcount; // if == validcount, already checked
 	polyobj_t *polyobj; // Belongs to a polyobject?
 
+	boolean tripwire;
+
 	char *text; // a concatenation of all front and back texture names, for linedef specials that require a string.
 	INT16 callcount; // no. of calls left before triggering, for the "X calls" linedef specials, defaults to 0
 } line_t;