T60748 Second try to fix start extreme in Noise modifier

The first point was moving inverted because first vector was inverted. Now, the vector is changed for the first point to get the same direction.

source/blender/gpencil_modifiers/intern/MOD_gpencilnoise.c

@@ -170,7 +170,12 @@ static void deformStroke(
 		}
 
 		/* initial vector (p0 -> p1) */
+		if (i == 0) {
+			sub_v3_v3v3(vec1, &pt0->x, &pt1->x);
+		}
+		else {
 			sub_v3_v3v3(vec1, &pt1->x, &pt0->x);
+		}
 		vran = len_v3(vec1);
 		/* vector orthogonal to normal */
 		cross_v3_v3v3(vec2, vec1, normal);