Implement park rendering at z4-5 (#1160)

Unblocks ZeLonewolf/openstreetmap-americana#51

Currently, OpenMapTiles renders protected areas at zoom 6 and higher.  However, the western United States has protected areas that are so vast that it makes sense to render these areas as low as zoom 4, as noted in ZeLonewolf/openstreetmap-americana#51

This PR extends the generalization of park features from its current termination at zoom 6 to zoom 4.

Here is an example from the Americana style of rendering protected areas at zoom 6, which is currently the lowest supported zoom:
https://zelonewolf.github.io/openstreetmap-americana/#6/45.313/-111.721

We would like to provide rendering at zoom 4 (no labels) and zoom 5 (with labels) in the Americana style.  Below are samples generated from this PR for Idaho, USA:

Zoom 4:
![image](https://user-images.githubusercontent.com/3254090/126924222-e21865e9-b184-479a-abd5-46238bd106f8.png)

Zoom 5:
![image](https://user-images.githubusercontent.com/3254090/126924230-01853e8c-d3e0-41e8-b10f-06415da529d8.png)

layers/park/park.sql

@@ -1,5 +1,5 @@
 -- etldoc: layer_park[shape=record fillcolor=lightpink, style="rounded,filled",
--- etldoc:     label="layer_park |<z6> z6 |<z7> z7 |<z8> z8 |<z9> z9 |<z10> z10 |<z11> z11 |<z12> z12|<z13> z13|<z14> z14+" ] ;
+-- etldoc:     label="layer_park |<z4> z4 |<z5> z5 |<z6> z6 |<z7> z7 |<z8> z8 |<z9> z9 |<z10> z10 |<z11> z11 |<z12> z12|<z13> z13|<z14> z14+" ] ;
 
 CREATE OR REPLACE FUNCTION layer_park(bbox geometry, zoom_level int, pixel_width numeric)
     RETURNS TABLE
@@ -37,6 +37,34 @@ FROM (
                 tags,
                 NULL::int AS rank
          FROM (
+                  -- etldoc: osm_park_polygon_dissolve_z4 -> layer_park:z4
+                  SELECT NULL::int AS osm_id,
+                         geometry,
+                         NULL AS name,
+                         NULL AS name_en,
+                         NULL AS name_de,
+                         NULL AS tags,
+                         NULL AS leisure,
+                         NULL AS boundary,
+                         NULL AS protection_title
+                  FROM osm_park_polygon_dissolve_z4
+                  WHERE zoom_level = 4
+                    AND geometry && bbox
+                  UNION ALL
+                  -- etldoc: osm_park_polygon_gen_z5 -> layer_park:z5
+                  SELECT osm_id,
+                         geometry,
+                         name,
+                         name_en,
+                         name_de,
+                         tags,
+                         leisure,
+                         boundary,
+                         protection_title
+                  FROM osm_park_polygon_gen_z5
+                  WHERE zoom_level = 5
+                    AND geometry && bbox
+                  UNION ALL
                   -- etldoc: osm_park_polygon_gen_z6 -> layer_park:z6
                   SELECT osm_id,
                          geometry,
@@ -184,6 +212,23 @@ FROM (
                         area DESC
                     )::int AS "rank"
          FROM (
+                  -- etldoc: osm_park_polygon_gen_z5 -> layer_park:z5
+                  SELECT osm_id,
+                         geometry_point,
+                         name,
+                         name_en,
+                         name_de,
+                         tags,
+                         leisure,
+                         boundary,
+                         protection_title,
+                         area
+                  FROM osm_park_polygon_gen_z5
+                  WHERE zoom_level = 5
+                    AND geometry_point && bbox
+                    AND area > 70000*2^(20-zoom_level)
+                  UNION ALL
+
                   -- etldoc: osm_park_polygon_gen_z6 -> layer_park:z6
                   SELECT osm_id,
                          geometry_point,