Add zoom, x, y and bbox variables to opts

Add some variables to be used later on by mapnik postgis input for token
substitution.

Note that the actual substitution depends on some changes in node-mapnik
and mapnik-vector-tile:

mapbox/mapnik-vector-tile#248
https://github.com/CartoDB/node-mapnik/blob/v3.6.2-carto/CHANGELOG.carto.md#362-carto1
CartoDB/node-mapnik#10

index.js

@@ -303,8 +303,14 @@ Bridge.getVector = function(source, map, z, x, y, callback) {
     // enable strictly_simple
     opts.strictly_simple = true;
 
-    // make zoom_level variable available to mapnik postgis datasource
-    opts.variables = { "zoom_level": z };
+    // make zoom, x, y and bbox variables available to mapnik postgis datasource
+    opts.variables = {
+        zoom_level: z, // for backwards compatibility
+        zoom: z,
+        x: x,
+        y: y,
+        bbox: JSON.stringify(map.extent)
+    };
 
     map.render(vtile, opts, function(err, vtile) {
         source._map.release(map);