fabricjs: fix setBackgroundImage

setBackgroundImage takes two mandatory args and an optional argument.

http://fabricjs.com/docs/fabric.StaticCanvas.html#setBackgroundImage

fabricjs/fabricjs-tests.ts

@@ -1056,4 +1056,10 @@ function sample8() {
             obj.setCoords();
         });
     };
-}
+}
+
+function sample9() {
+    var canvas = new fabric.Canvas('c');
+    canvas.setBackgroundImage('yolo.jpg', () => {"a"}, {opacity: 45});
+    canvas.setBackgroundImage('yolo.jpg', () => {"a"});
+}

fabricjs/fabricjs.d.ts

@@ -546,7 +546,7 @@ declare module fabric {
 
         sendBackwards(object: IObject): ICanvas;
         sendToBack(object: IObject): ICanvas;
-        setBackgroundImage(object: IObject): ICanvas;
+        setBackgroundImage(image: any, callback: () => any, options?): ICanvas;
         setDimensions(object: { width: number; height: number; }): ICanvas;
         setHeight(height: number): ICanvas;
         setOverlayImage(url: string, callback: () => any, options): ICanvas;