Add None tile layer

Fixes #1472

data/core.yaml

@@ -211,6 +211,7 @@ en:
     title: Background
     description: Background settings
     percent_brightness: "{opacity}% brightness"
+    none: None
     custom: Custom
     custom_prompt: "Enter a tile template. Valid tokens are {z}, {x}, {y} for Z/X/Y scheme and {u} for quadtile scheme."
     fix_misalignment: Fix misalignment

dist/locales/en.json

@@ -260,6 +260,7 @@
         "title": "Background",
         "description": "Background settings",
         "percent_brightness": "{opacity}% brightness",
+        "none": "None",
         "custom": "Custom",
         "custom_prompt": "Enter a tile template. Valid tokens are {z}, {x}, {y} for Z/X/Y scheme and {u} for quadtile scheme.",
         "fix_misalignment": "Fix misalignment",

js/id/renderer/background.js

@@ -14,6 +14,8 @@ iD.Background = function(context) {
         }
     });
 
+    backgroundSources.unshift(iD.BackgroundSource.None());
+
     function findSource(id) {
         return _.find(backgroundSources, function(d) {
             return d.id && d.id === id;

js/id/renderer/background_source.js

@@ -112,3 +112,7 @@ iD.BackgroundSource.Bing = function(data, dispatch) {
 
     return bing;
 };
+
+iD.BackgroundSource.None = function() {
+    return iD.BackgroundSource({ name: t('background.none'), id: 'None', template: '' });
+};

js/id/renderer/tile_layer.js

@@ -68,6 +68,7 @@ iD.TileLayer = function() {
         if (source.validZoom(z)) {
             tile().forEach(function(d) {
                 addSource(d);
+                if (d[3] === '') return;
                 requests.push(d);
                 if (cache[d[3]] === false && lookUp(d)) {
                     requests.push(addSource(lookUp(d)));