Add default "Locator Overlay" layer

A layer that is by default visible at <z16 and can
orient you with major roads, cities, borders, etc.

Fixes #1301

data/imagery.json

@@ -13,12 +13,29 @@
             "2",
             "3"
         ],
-        "default": "yes",
+        "default": true,
         "sourcetag": "Bing",
         "logo": "bing_maps.png",
         "logo_url": "http://www.bing.com/maps",
         "terms_url": "http://opengeodata.org/microsoft-imagery-details"
     },
+    {
+        "name": "Locator Overlay",
+        "template": "http://{t}.tiles.mapbox.com/v3/openstreetmap.map-btyhiati/{z}/{x}/{y}.png",
+        "description": "Shows major features to help orient you.",
+        "overlay": true,
+        "default": true,
+        "scaleExtent": [
+            0,
+            16
+        ],
+        "subdomains": [
+            "a",
+            "b",
+            "c"
+        ],
+        "terms_url": "http://mapbox.com/tos/"
+    },
     {
         "name": "MapBox Satellite",
         "template": "http://{t}.tiles.mapbox.com/v3/openstreetmap.map-4wvf9l0l/{z}/{x}/{y}.png",

js/id/renderer/background.js

@@ -196,6 +196,14 @@ iD.Background = function(context) {
         background.baseLayerSource(findSource(chosen) || findSource("Bing"));
     }
 
+    var locator = _.find(backgroundSources, function(d) {
+        return d.overlay && d.default;
+    });
+
+    if (locator) {
+        background.toggleOverlayLayer(locator);
+    }
+
     var overlays = (q.overlays || '').split(',');
     overlays.forEach(function(overlay) {
         overlay = findSource(overlay);

js/id/renderer/background_source.js

@@ -49,6 +49,11 @@ iD.BackgroundSource = function(data) {
         });
     };
 
+    source.validZoom = function(z) {
+        return source.scaleExtent[0] <= z &&
+            (source.name !== 'Locator Overlay' || source.scaleExtent[1] > z);
+    };
+
     source.copyrightNotices = function() {};
 
     return source;

js/id/renderer/tile_layer.js

@@ -65,7 +65,7 @@ iD.TileLayer = function() {
     function render(selection) {
         var requests = [];
 
-        if (tile.scaleExtent()[0] <= z) {
+        if (source.validZoom(z)) {
             tile().forEach(function(d) {
                 addSource(d);
                 requests.push(d);