Change boundary nature reserve / national park areas

Render nature reserve / national park areas with a double uninterruped line,
like marinas are currently rendered.

* Resolves #69
* Resolves #563

admin.mss

@@ -87,21 +87,28 @@
 }
 
 #nature-reserve-boundaries {
-  [zoom >= 7] {
+  [zoom >= 7][way_pixels > 100] {
     ::fill [zoom < 13] {
       opacity: 0.05;
       polygon-fill: green;
     }
-    ::line {
-      opacity: 0.15;
-      line-color: green;
-      line-width: 1.5;
-      line-dasharray: 4,2;
-      [zoom >= 10] {
-        line-width: 3;
-        line-dasharray: 6,2;
-        line-join: bevel;
-      }
+    a/line-width: 1;
+    a/line-offset: -0.5;
+    a/line-color: green;
+    a/line-opacity: 0.15;
+    a/line-join: round;
+    a/line-cap: round;
+    b/line-width: 3;
+    b/line-offset: -1.5;
+    b/line-color: green;
+    b/line-opacity: 0.15;
+    b/line-join: round;
+    b/line-cap: round;
+    [zoom >= 14] {
+      a/line-width: 2;
+      a/line-offset: -1;
+      b/line-width: 6;
+      b/line-offset: -3;
     }
   }
 }

project.mml

@@ -1653,7 +1653,7 @@
       "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over", 
       "Datasource": {
         "extent": "-20037508,-20037508,20037508,20037508", 
-        "table": "(SELECT way, way_area, name, boundary FROM planet_osm_polygon WHERE (boundary = 'national_park' OR leisure = 'nature_reserve') AND building IS NULL) AS national_park_boundaries", 
+        "table": "(SELECT way, way_area, name, boundary, way_area/(!pixel_width!*!pixel_height!) AS way_pixels FROM planet_osm_polygon WHERE (boundary = 'national_park' OR leisure = 'nature_reserve') AND building IS NULL) AS national_park_boundaries", 
         "geometry_field": "way", 
         "type": "postgis", 
         "key_field": "", 

project.yaml

@@ -1504,7 +1504,7 @@ Layer:
     Datasource:
       <<: *osm2pgsql
       table: |2-
-        (SELECT way, way_area, name, boundary FROM planet_osm_polygon WHERE (boundary = 'national_park' OR leisure = 'nature_reserve') AND building IS NULL) AS national_park_boundaries
+        (SELECT way, way_area, name, boundary, way_area/(!pixel_width!*!pixel_height!) AS way_pixels FROM planet_osm_polygon WHERE (boundary = 'national_park' OR leisure = 'nature_reserve') AND building IS NULL) AS national_park_boundaries
     advanced: {}
   - id: "theme-park"
     name: "theme-park"