RNAV: use runway departure end for previous wpt.

Should fix leg computation following a runway waypt.

src/Instrumentation/gps.cxx

@@ -464,9 +464,16 @@ SGGeod GPS::previousLegWaypointPosition(bool& isValid)
 		Waypt* waypt = leg->waypoint();
 		if (waypt) {
             isValid = true;
+            // ensure computations use runway end, not threshold
+            if (waypt->type() == "runway") {
+                RunwayWaypt* rwpt = static_cast<RunwayWaypt*>(waypt);
+                return rwpt->runway()->end();
+            }
+
 			return waypt->position();
 		}
 	}
+
 	isValid = false;
     return SGGeod();
 }