Honor windows.devicePixelRatio by default

Cesium now honors `window.devicePixelRatio` by default, which greatly
improving performance on mobile devices and high DPI displays by rendering
at the browser-recommended resolution.

This change also decreases bandwidth usage because we request far fewer
tiles when rendering at lower resolutions.

CHANGES.md

@@ -33,6 +33,7 @@ Change Log
 * Added `createOpenStreetMapImageryProvider` function to replace the `OpenStreetMapImageryProvider` class. This function returns a constructed `UrlTemplateImageryProvider`.
 * Fixed an issue with tile selection when below the surface of the ellipsoid. [#3170](https://github.com/AnalyticalGraphicsInc/cesium/issues/3170)
 * Fixed an issue with loading skeletons for skinned glTF models. [#3224](https://github.com/AnalyticalGraphicsInc/cesium/pull/3224)
+* Cesium now honors `window.devicePixelRatio` by default, which greatly improving performance on mobile devices and high DPI displays by rendering at the browser-recommended resolution. To use the previous behavior, set `Viewer.resolutionScale` to `window.devicePixelRatio`.
 
 ### 1.15 - 2015-11-02
 

Source/Widgets/CesiumWidget/CesiumWidget.js

@@ -98,7 +98,7 @@ define([
         var canvas = widget._canvas;
         var width = canvas.clientWidth;
         var height = canvas.clientHeight;
-        var zoomFactor = defaultValue(window.devicePixelRatio, 1.0) * widget._resolutionScale;
+        var zoomFactor = (1.0 / defaultValue(window.devicePixelRatio, 1.0)) * widget._resolutionScale;
 
         widget._canvasWidth = width;
         widget._canvasHeight = height;