Allow for access to imageData on L.ImageOverlay

This implements the changes in https://github.com/Leaflet/Leaflet/pull/2621 for cross origin tiles for `L.ImageOverlay` so that the image data can be accessed in canvas and saved.

src/layer/ImageOverlay.js

@@ -8,6 +8,10 @@ L.ImageOverlay = L.Layer.extend({
 		opacity: 1,
 		alt: '',
 		interactive: false
+
+		/*
+		crossOrigin: <Boolean>,
+		*/
 	},
 
 	initialize: function (url, bounds, options) { // (String, LatLngBounds, Object)
@@ -114,6 +118,11 @@ L.ImageOverlay = L.Layer.extend({
 		img.onmousemove = L.Util.falseFn;
 
 		img.onload = L.bind(this.fire, this, 'load');
+
+		if (this.options.crossOrigin) {
+			img.crossOrigin = '';
+		}
+
 		img.src = this._url;
 		img.alt = this.options.alt;
 	},