COrrectly compute contour length for wrapped contours (#6102)

COntours split across periodic boundaries will have jumps across the map.  These are not drawn, but the code used them in computing the length which affects how many ticks are set.  This corrects the contour length calculations.

Author: PaulWessel

src/grdcontour.c

@@ -540,7 +540,7 @@ GMT_LOCAL void grdcontour_sort_and_plot_ticks (struct GMT_CTRL *GMT, struct PSL_
 	double add, dx, dy, x_back, y_back, x_front, y_front, x_end, y_end, x_lbl, y_lbl;
 	double xmin, xmax, ymin, ymax, inc, dist, a, this_lon, this_lat, sa, ca;
 	double *s = NULL, *xp = NULL, *yp = NULL;
-	double da, db, dc, dd;
+	double da, db, dc, dd, L, L_cut;
 
 	double tick_gap = I->dim[GMT_X];
 	double tick_length = I->dim[GMT_Y];
@@ -668,8 +668,13 @@ GMT_LOCAL void grdcontour_sort_and_plot_ticks (struct GMT_CTRL *GMT, struct PSL_
 		if (np == 0) continue;
 
 		s = gmt_M_memory (GMT, NULL, np, double);	/* Compute distance along the contour */
-		for (j = 1, s[0] = 0.0; j < np; j++)
-			s[j] = s[j-1] + hypot (xp[j]-xp[j-1], yp[j]-yp[j-1]);
+		for (j = 1, s[0] = 0.0; j < np; j++) {
+			L = hypot (xp[j]-xp[j-1], yp[j]-yp[j-1]);
+			L_cut = gmt_half_map_width (GMT, 0.5 * (yp[j]+yp[j-1]));
+			if (L > L_cut)	/* Eliminate horizontal jumps exceeding half mapwidth at this y-value */
+				L = 0.0;
+			s[j] = s[j-1] + L;
+		}
 		n_ticks = irint (floor (s[np-1] / tick_gap));
 		if (s[np-1] < GRDCONTOUR_MIN_LENGTH || n_ticks == 0) {	/* Contour is too short to be ticked or labeled */
 			save[pol].do_it = false;