Order amenity_lines by layer (gravitystorm#3413).

This way, we respect the layer tag for water slides, and it should not harm for fords.

project.mml

@@ -1564,6 +1564,7 @@ Layer:
         (SELECT
           way,
           name,
+          layer,
           COALESCE(
            'highway_' || CASE WHEN tags @> 'ford=>yes' OR tags @> 'ford=>stepping_stones' THEN 'ford' ELSE NULL END,
            'leisure_' || CASE WHEN leisure IN ('slipway') THEN leisure ELSE NULL END,
@@ -1574,6 +1575,7 @@ Layer:
           WHERE tags @> 'ford=>yes' OR tags @> 'ford=>stepping_stones'
             OR leisure IN ('slipway')
             OR tags @> 'attraction=>water_slide'
+          ORDER BY COALESCE(layer,0)
         ) AS amenity_line
     properties:
       minzoom: 16