Consolidate code for determining the pixel ratio

lib/network/modules/Canvas.js

@@ -325,17 +325,30 @@ class Canvas {
 
 
   /**
-   * @private
+   * Determine the pixel ratio for various browsers.
+   *
+   * Note that there is no dependency on local context; this might as well be
+   * a static method.
+   *
+   * Also called in CanvasRenderer.
    */
-  _setPixelRatio(ctx) {
-    this.pixelRatio = (window.devicePixelRatio || 1) / (ctx.webkitBackingStorePixelRatio ||
+  _getPixelRatio(ctx) {
+    return (window.devicePixelRatio || 1) / (ctx.webkitBackingStorePixelRatio ||
       ctx.mozBackingStorePixelRatio ||
       ctx.msBackingStorePixelRatio  ||
       ctx.oBackingStorePixelRatio   ||
       ctx.backingStorePixelRatio    || 1);
   }
 
 
+  /**
+   * @private
+   */
+  _setPixelRatio(ctx) {
+    this.pixelRatio = this._getPixelRatio(ctx);
+  }
+
+
   /**
    * Convert the X coordinate in DOM-space (coordinate point in browser relative to the container div) to
    * the X coordinate in canvas-space (the simulation sandbox, which the camera looks upon)

lib/network/modules/CanvasRenderer.js

@@ -145,12 +145,7 @@ class CanvasRenderer {
         this.canvas.setSize();
       }
 
-      this.pixelRatio = (window.devicePixelRatio || 1) / (ctx.webkitBackingStorePixelRatio ||
-        ctx.mozBackingStorePixelRatio ||
-        ctx.msBackingStorePixelRatio ||
-        ctx.oBackingStorePixelRatio ||
-        ctx.backingStorePixelRatio || 1);
-
+      this.pixelRatio = this.canvas._getPixelRatio(ctx);
       ctx.setTransform(this.pixelRatio, 0, 0, this.pixelRatio, 0, 0);
 
       // clear the canvas
@@ -206,11 +201,7 @@ class CanvasRenderer {
   _resizeNodes() {
     let ctx = this.canvas.frame.canvas.getContext('2d');
     if (this.pixelRatio === undefined) {
-      this.pixelRatio = (window.devicePixelRatio || 1) / (ctx.webkitBackingStorePixelRatio ||
-        ctx.mozBackingStorePixelRatio ||
-        ctx.msBackingStorePixelRatio ||
-        ctx.oBackingStorePixelRatio ||
-        ctx.backingStorePixelRatio || 1);
+      this.pixelRatio = this.canvas._getPixelRatio(ctx);
     }
     ctx.setTransform(this.pixelRatio, 0, 0, this.pixelRatio, 0, 0);
     ctx.save();