Selective rendering of tracks and paths at z12-13 (#1190)

Closes #271

This PR adds track and path rendering at lower zooms than currently provided, and also achieves near-parity with openstreetmap-carto on track and path rendering. A previously-abandoned attempt, with significant discussion, was #1169.

layers/transportation/mapping.yaml

@@ -151,6 +151,10 @@ mtb_scale_field: &mtb_scale
   key: mtb:scale
   name: mtb_scale
   type: string
+sac_scale_field: &sac_scale
+  key: sac_scale
+  name: sac_scale
+  type: string
 surface_field: &surface
   key: surface
   name: surface
@@ -200,6 +204,7 @@ tables:
     - *foot
     - *horse
     - *mtb_scale
+    - *sac_scale
     - *surface
     mapping:
       highway:
@@ -425,6 +430,13 @@ tables:
     - *ref
     - *network
     - *name
+    - name: osmc_symbol
+      key: osmc:symbol
+      type: string
+    - name: colour
+      key: colour
+      type: string
     mapping:
       route:
       - road
+      - hiking