Fixing ferry routes over variable color water.

This is not a perfect solution and depends on river color being
the darkest but it seems to work all right even on ocean.

ferry-routes.mss

@@ -4,8 +4,10 @@
 #ferry-routes {
   [zoom >= 7] {
     /* background prevents problems with overlapping ferry-routes, see #457 */
-    background/line-color: @water-color;
+    background/line-color: @river-color;
     background/line-width: 1; /* Needs to be a bit wider than the route itself because of antialiasing */
+    background/line-comp-op: lighten;
+    background/line-dasharray: 4,4;
     line-color: @ferry-route;
     line-width: 0.4;
     line-dasharray: 4,4;