Add support for runway=displaced_threshold

Stop rendering its `ref` (this is better left to the main body of the
`aeroway=runway` and draw the arrows that are typically used to mark a
displaced threshold in aviation.

project.mml

@@ -1035,6 +1035,7 @@ Layer:
         (SELECT
             way,
             aeroway,
+            runway,
             bridge,
             width,
             width::real/sf AS area_width,
@@ -1043,6 +1044,7 @@ Layer:
           (SELECT
               way,
               aeroway,
+              tags->'runway' AS runway,
               bridge IN ('yes', 'boardwalk', 'cantilever', 'covered', 'low_water_crossing', 'movable', 'trestle', 'viaduct') AS bridge,
               -- Convert valid length units into metres. Fall back to a sensible default if no valid value was set.
               GREATEST(
@@ -1800,13 +1802,20 @@ Layer:
                 SELECT
                     osm_id,
                     way,
-                    COALESCE(
-                      CASE WHEN highway IN ('motorway', 'trunk', 'primary', 'secondary', 'tertiary') THEN highway END,
-                      CASE WHEN aeroway IN ('runway', 'taxiway') THEN aeroway END
-                    ) AS highway,
+                    highway,
+                    string_to_array(ref, ';') AS refs
+                  FROM planet_osm_line
+                  WHERE highway IN ('motorway', 'trunk', 'primary', 'secondary', 'tertiary')
+                    AND ref IS NOT NULL
+                UNION ALL
+                SELECT
+                    osm_id,
+                    way,
+                    aeroway AS highway,
                     string_to_array(ref, ';') AS refs
                   FROM planet_osm_line
-                  WHERE (highway IN ('motorway', 'trunk', 'primary', 'secondary', 'tertiary') OR aeroway IN ('runway', 'taxiway'))
+                  WHERE aeroway IN ('runway', 'taxiway')
+                    AND NOT tags @> '"runway"=>"displaced_threshold"'
                     AND ref IS NOT NULL
               ) AS p) AS q
           WHERE height <= 4 AND width <= 11

style/roads.mss

@@ -2991,6 +2991,14 @@ tertiary is rendered from z10 and is not included in osm_planet_roads. */
         [zoom >= 18] { line-dasharray: 84,56; }
         [zoom >= 19] { line-dasharray: 168,112; }
         [zoom >= 20] { line-dasharray: 336,224; }
+        [runway = "displaced_threshold"] {
+          line-width: 0;
+          marker-placement: line;
+          marker-fill: white;
+          marker-width: [line_width] * 16;
+          marker-spacing: [line_width] * 50;
+          marker-file: url('symbols/displaced_threshold.svg');
+        }
       }
     }
   }