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/mapping.yaml

@@ -1,33 +1,45 @@
 generalized_tables:
+  # etldoc: osm_park_polygon_gen_z5 -> osm_park_polygon_gen_z4
+  park_polygon_gen_z4:
+    source: park_polygon_gen_z5
+    sql_filter: area>power(ZRES3,2)
+    tolerance: ZRES4
+
+  # etldoc: osm_park_polygon_gen_z6 -> osm_park_polygon_gen_z5
+  park_polygon_gen_z5:
+    source: park_polygon_gen_z6
+    sql_filter: area>power(ZRES4,2)
+    tolerance: ZRES5
+
   # etldoc: osm_park_polygon_gen_z7 -> osm_park_polygon_gen_z6
   park_polygon_gen_z6:
     source: park_polygon_gen_z7
     sql_filter: area>power(ZRES5,2)
-    tolerance: ZRES8
+    tolerance: ZRES6
 
   # etldoc: osm_park_polygon_gen_z8 -> osm_park_polygon_gen_z7
   park_polygon_gen_z7:
     source: park_polygon_gen_z8
     sql_filter: area>power(ZRES6,2)
-    tolerance: ZRES8
+    tolerance: ZRES7
 
   # etldoc: osm_park_polygon_gen_z9 -> osm_park_polygon_gen_z8
   park_polygon_gen_z8:
     source: park_polygon_gen_z9
     sql_filter: area>power(ZRES7,2)
-    tolerance: ZRES9
+    tolerance: ZRES8
 
   # etldoc: osm_park_polygon_gen_z10 -> osm_park_polygon_gen_z9
   park_polygon_gen_z9:
     source: park_polygon_gen_z10
     sql_filter: area>power(ZRES8,2)
-    tolerance: ZRES10
+    tolerance: ZRES9
 
   # etldoc: osm_park_polygon_gen_z11 -> osm_park_polygon_gen_z10
   park_polygon_gen_z10:
     source: park_polygon_gen_z11
     sql_filter: area>power(ZRES9,2)
-    tolerance: ZRES11
+    tolerance: ZRES10
 
   # etldoc: osm_park_polygon_gen_z12 -> osm_park_polygon_gen_z11
   park_polygon_gen_z11: