Filter out small areas and use polygon-gamma for large ones

This should fill in gaps between water bodies, prevent colouring bugs
and not over-represent water at low zooms

project.mml

@@ -213,7 +213,7 @@
       ],
       "Datasource": {
         "type": "postgis",
-        "table": "(SELECT\n    way, \"natural\", waterway, landuse, name\n  FROM planet_osm_polygon\n  WHERE\n    (waterway IN ('dock', 'riverbank', 'canal')\n      OR landuse IN ('reservoir','basin')\n      OR \"natural\" IN ('lake','water','land','glacier','mud'))\n    AND building IS NULL\n  ORDER BY z_order, way_area) AS water_areas",
+        "table": "(SELECT\n    way, \"natural\", waterway, landuse, name, way_area/(!pixel_width!*!pixel_height!) AS way_pixels\n  FROM planet_osm_polygon\n  WHERE\n    (waterway IN ('dock', 'riverbank', 'canal')\n      OR landuse IN ('reservoir','basin')\n      OR \"natural\" IN ('lake','water','land','glacier','mud'))\n    AND building IS NULL\n    AND way_area/(!pixel_width!*!pixel_height!) > 0.01\n  ORDER BY z_order, way_area) AS water_areas",
         "extent": "-20037508,-20037508,20037508,20037508",
         "key_field": "",
         "geometry_field": "way",

water.mss

@@ -17,11 +17,17 @@
   [waterway = 'canal'] {
     [zoom >= 9]::waterway {
       polygon-fill: @water-color;
+      [way_pixels >= 4] {
+        polygon-gamma: 0.75;
+      }
     }
   }
 
   [landuse = 'basin'][zoom >= 7]::landuse {
     polygon-fill: @water-color;
+    [way_pixels >= 4] {
+      polygon-gamma: 0.75;
+    }
   }
 
   [natural = 'lake']::natural,
@@ -30,6 +36,9 @@
   [waterway = 'riverbank']::waterway {
     [zoom >= 6] {
       polygon-fill: @water-color;
+      [way_pixels >= 4] {
+        polygon-gamma: 0.75;
+      }
     }
   }