Fix dropshadows of papersprites drifting depending on angle relative to...

Fix dropshadows of papersprites drifting depending on angle relative to camera. Discovered in Kart internal for the paper item drops and ported back, hence the branch name.

src/r_things.c

@@ -1423,7 +1423,7 @@ static void R_ProjectSprite(mobj_t *thing)
 
 	fixed_t sheartan = 0;
 	fixed_t shadowscale = FRACUNIT;
-	fixed_t basetx; // drop shadows
+	fixed_t basetx, basetz; // drop shadows
 
 	boolean shadowdraw, shadoweffects, shadowskew;
 	boolean splat = R_ThingIsFloorSprite(thing);
@@ -1453,7 +1453,7 @@ static void R_ProjectSprite(mobj_t *thing)
 	tr_x = thing->x - viewx;
 	tr_y = thing->y - viewy;
 
-	tz = FixedMul(tr_x, viewcos) + FixedMul(tr_y, viewsin); // near/far distance
+	basetz = tz = FixedMul(tr_x, viewcos) + FixedMul(tr_y, viewsin); // near/far distance
 
 	// thing is behind view plane?
 	if (!papersprite && (tz < FixedMul(MINZ, this_scale))) // papersprite clipping is handled later
@@ -2052,7 +2052,7 @@ static void R_ProjectSprite(mobj_t *thing)
 		R_SplitSprite(vis);
 
 	if (oldthing->shadowscale && cv_shadow.value)
-		R_ProjectDropShadow(oldthing, vis, oldthing->shadowscale, basetx, tz);
+		R_ProjectDropShadow(oldthing, vis, oldthing->shadowscale, basetx, basetz);
 
 	// Debug
 	++objectsdrawn;