Allow retina Mapbox imagery and other formats to be specified

This just allows `format` to be anything, rather than forcing it to start
with a `.` which provides the flexibility for specifying `@2x.png`.

Replaces #3899.

Author: mramato

CHANGES.md

@@ -14,6 +14,7 @@ Change Log
 * Fixed a bug with rotated, textured rectangles. [#4430](https://github.com/AnalyticalGraphicsInc/cesium/pull/4430)
 * Fixed a bug when morphing from 2D to 3D. [#4388](https://github.com/AnalyticalGraphicsInc/cesium/pull/4388)
 * Added `Rectangle.simpleIntersection`.
+* Added the ability to specify retina options, such as `@2x.png`, via the `MapboxImageryProvider` `format` option.
 * Removed an unnecessary reprojection of Web Mercator imagery tiles to the Geographic projection on load.  This should improve both visual quality and load performance slightly.
 
 ### 1.26 - 2016-10-03

Source/Scene/MapboxImageryProvider.js

@@ -66,13 +66,16 @@ define([
         this._mapId = mapId;
         this._accessToken = MapboxApi.getAccessToken(options.accessToken);
         var format = defaultValue(options.format, 'png');
-        this._format = format.replace('.', '');
+        if (!/\./.test(format)) {
+            format = '.' + format;
+        }
+        this._format = format;
 
         var templateUrl = url;
         if (!trailingSlashRegex.test(url)) {
             templateUrl += '/';
         }
-        templateUrl += mapId + '/{z}/{x}/{y}.' + this._format;
+        templateUrl += mapId + '/{z}/{x}/{y}' + this._format;
         if (defined(this._accessToken)) {
             templateUrl += '?access_token=' + this._accessToken;
         }

Specs/Scene/MapboxImageryProviderSpec.js

@@ -280,4 +280,50 @@ defineSuite([
             });
         });
     });
+
+    it('appends specified format', function() {
+        var provider = new MapboxImageryProvider({
+            url : 'made/up/mapbox/server/',
+            mapId: 'test-id',
+            format: '@2x.png'
+        });
+
+        return pollToPromise(function() {
+            return provider.ready;
+        }).then(function() {
+            spyOn(loadImage, 'createImage').and.callFake(function(url, crossOrigin, deferred) {
+                expect(/made\/up\/mapbox\/server\/test-id\/0\/0\/0@2x\.png\?access_token=/.test(url)).toBe(true);
+
+                // Just return any old image.
+                loadImage.defaultCreateImage('Data/Images/Red16x16.png', crossOrigin, deferred);
+            });
+
+            return provider.requestImage(0, 0, 0).then(function(image) {
+                expect(loadImage.createImage).toHaveBeenCalled();
+            });
+        });
+    });
+
+    it('adds missing period for format', function() {
+        var provider = new MapboxImageryProvider({
+            url : 'made/up/mapbox/server/',
+            mapId: 'test-id',
+            format: 'png'
+        });
+
+        return pollToPromise(function() {
+            return provider.ready;
+        }).then(function() {
+            spyOn(loadImage, 'createImage').and.callFake(function(url, crossOrigin, deferred) {
+                expect(/made\/up\/mapbox\/server\/test-id\/0\/0\/0\.png\?access_token=/.test(url)).toBe(true);
+
+                // Just return any old image.
+                loadImage.defaultCreateImage('Data/Images/Red16x16.png', crossOrigin, deferred);
+            });
+
+            return provider.requestImage(0, 0, 0).then(function(image) {
+                expect(loadImage.createImage).toHaveBeenCalled();
+            });
+        });
+    });
 });