Fix accessing webkitBackingStorePixelRatio of context on iOS10 (#26)

On iOS 10 webkitBackingStorePixelRatio can be accessed only by using
instance of CanvasRenderingContext2D. Using Object.create(prototype)
in that case doesn't work so the only way to access that property is by
getting context 2d from canvas element.

.gitignore

@@ -1 +1,2 @@
 /node_modules
+npm-debug.log

dist/hidpi-canvas.js

@@ -8,16 +8,18 @@
 */
 (function(prototype) {
 
-	var pixelRatio = (function(context) {
-			var backingStore = context.backingStorePixelRatio ||
-						context.webkitBackingStorePixelRatio ||
-						context.mozBackingStorePixelRatio ||
-						context.msBackingStorePixelRatio ||
-						context.oBackingStorePixelRatio ||
-						context.backingStorePixelRatio || 1;
+	var pixelRatio = (function() {
+			var canvas = document.createElement('canvas'),
+				context = canvas.getContext('2d'),
+				backingStore = context.backingStorePixelRatio ||
+					context.webkitBackingStorePixelRatio ||
+					context.mozBackingStorePixelRatio ||
+					context.msBackingStorePixelRatio ||
+					context.oBackingStorePixelRatio ||
+					context.backingStorePixelRatio || 1;
 
 			return (window.devicePixelRatio || 1) / backingStore;
-		})(prototype),
+		})(),
 
 		forEach = function(obj, func) {
 			for (var p in obj) {

src/CanvasRenderingContext2D.js

@@ -1,15 +1,17 @@
 (function(prototype) {
 
-	var pixelRatio = (function(context) {
-			var backingStore = context.backingStorePixelRatio ||
-						context.webkitBackingStorePixelRatio ||
-						context.mozBackingStorePixelRatio ||
-						context.msBackingStorePixelRatio ||
-						context.oBackingStorePixelRatio ||
-						context.backingStorePixelRatio || 1;
+	var pixelRatio = (function() {
+			var canvas = document.createElement('canvas'),
+				context = canvas.getContext('2d'),
+				backingStore = context.backingStorePixelRatio ||
+					context.webkitBackingStorePixelRatio ||
+					context.mozBackingStorePixelRatio ||
+					context.msBackingStorePixelRatio ||
+					context.oBackingStorePixelRatio ||
+					context.backingStorePixelRatio || 1;
 
 			return (window.devicePixelRatio || 1) / backingStore;
-		})(prototype),
+		})(),
 
 		forEach = function(obj, func) {
 			for (var p in obj) {