Add color fill for mangrove, reedbed and saltmarsh wetlands (#3807)

* Add salt dots pattern to saltmarsh fill color

* Add fill for mangroves, reedbeds and saltmarsh

Mangroves: scrub color - reedbed: grass color - saltmarsh: grass color

landcover.mss

@@ -539,6 +539,18 @@
     [way_pixels >= 64] { polygon-gamma: 0.3;  }
   }
 
+  [feature = 'wetland_mangrove'][zoom >= 5] {
+    polygon-fill: @scrub;
+    [way_pixels >= 4]  { polygon-gamma: 0.75; }
+    [way_pixels >= 64] { polygon-gamma: 0.3;  }
+  }
+
+  [feature = 'wetland_reedbed'][zoom >= 5] {
+    polygon-fill: @grass;
+    [way_pixels >= 4]  { polygon-gamma: 0.75; }
+    [way_pixels >= 64] { polygon-gamma: 0.3;  }
+  }
+
   [feature = 'wetland_bog'],
   [feature = 'wetland_string_bog'] {
     [zoom >= 5] {
@@ -550,9 +562,16 @@
 
   [feature = 'wetland_wet_meadow'],
   [feature = 'wetland_fen'],
+  [feature = 'wetland_saltmarsh'],
   [feature = 'wetland_marsh'] {
     [zoom >= 5] {
       polygon-fill: @grass;
+      [feature = 'wetland_saltmarsh'][zoom >= 13] {
+        polygon-pattern-file: url('symbols/salt-dots-2.png');
+        polygon-pattern-alignment: global;
+        [way_pixels >= 4]  { polygon-pattern-gamma: 0.75; }
+        [way_pixels >= 64] { polygon-pattern-gamma: 0.3;  }
+      }
       [way_pixels >= 4]  { polygon-gamma: 0.75; }
       [way_pixels >= 64] { polygon-gamma: 0.3;  }
     }

symbols/generating_patterns/salt-dots.md

@@ -0,0 +1,11 @@
+The salt-dots pattern is based on salt-dots.svg
+
+For the pattern visit [jsdotpattern (command sequence is recorded)](http://www.imagico.de/map/jsdotpattern.php#x,256,jdp68152;g,5,32,32;rx,25,2,32,32;rx,25,2,32,32;rx,25,2,32,32;rd,0,0,0,dot,0.125,10,9,0,jdp48264,ffffff,cdebb0;)
+
+The result is this following [pattern](http://www.imagico.de/map/jsdotpattern.php#x,256,jdp68152;g,5,32,32;rx,25,2,32,32;rx,25,2,32,32;rx,25,2,32,32;rd,0,0,0,dot,0.125,10,9,0,jdp48264,ffffff,cdebb0;).
+
+Generated SVG image is sanitized for use with Mapnik by the script svg_pattern.sh from the jsdotpattern repository at http://github.com/imagico/jsdotpattern.
+
+The final file is salt-dots.svg
+
+However, since SVG patterns is not properly handled by Mapnik at this time, the SVG file was converted to a PNG file with alpha channel, salt-dots.png