Revamp water label display logic (#1457)

* At zooms 9-13, water labels will now be rendered for water features that are at least 0.25 tiles in size.  This means that water features that are the same size as a 128x128 pixel square will get a point label.
* Labels for large water features are now shown at the lowest zoom at which it's appropriate to show a label.
* Bay and strait names (`natural=bay` and `natural=strait`) are included. 
* Large seas modeled as areas (e.g. Caspian Sea) render as `class=sea` rather than `class=lake`

layers/water_name/mapping.yaml

@@ -21,6 +21,9 @@ tables:
     - name: place
       key: place
       type: string
+    - name: natural
+      key: natural
+      type: string
     - name: rank
       key: rank
       type: integer
@@ -34,3 +37,6 @@ tables:
       place:
       - ocean
       - sea
+      natural:
+      - bay
+      - strait