Refactor z12-14 transportation filter logic (#1207)

This PR is a refactor.  The purpose of this PR is to refactor the logic in the z12-z14 transportation layer to increase readability and reduce code complexity.

In addition, this fixes bugs in the original filter code:
* Create filter parity between `service` and `service_construction` highway classes.
* Removal of unintended `class` values `minor_construction`, `path_construction`, `service_construction`, and `track_construction` from zoom 12 filter by explicitly listing the highway classes that are included.

layers/transportation/class.sql

@@ -56,3 +56,37 @@ SELECT CASE
 $$ LANGUAGE SQL IMMUTABLE
                 STRICT
                 PARALLEL SAFE;
+
+-- Determine which transportation features are shown at zoom 12
+CREATE OR REPLACE FUNCTION transportation_filter_z12(highway text, construction text) RETURNS boolean AS
+$$
+SELECT CASE
+           WHEN highway IN ('unclassified', 'residential') THEN TRUE
+           WHEN highway_class(highway, '', construction) IN
+               (
+                'motorway', 'trunk', 'primary', 'secondary', 'tertiary', 'raceway',
+                'motorway_construction', 'trunk_construction', 'primary_construction',
+                'secondary_construction', 'tertiary_construction', 'raceway_construction'
+               ) THEN TRUE --includes ramps
+           ELSE FALSE
+       END
+$$ LANGUAGE SQL IMMUTABLE
+                STRICT
+                PARALLEL SAFE;
+
+-- Determine which transportation features are shown at zoom 13
+-- Assumes that piers have already been excluded
+CREATE OR REPLACE FUNCTION transportation_filter_z13(highway text,
+                                                     public_transport text,
+                                                     construction text,
+                                                     service text) RETURNS boolean AS
+$$
+SELECT CASE
+           WHEN transportation_filter_z12(highway, construction) THEN TRUE
+           WHEN highway = 'service' OR construction = 'service' THEN service NOT IN ('driveway', 'parking_aisle')
+           WHEN highway_class(highway, public_transport, construction) IN ('minor', 'minor_construction') THEN TRUE
+           ELSE FALSE
+       END
+$$ LANGUAGE SQL IMMUTABLE
+                STRICT
+                PARALLEL SAFE;