Examples: Add comment for offscreen canvas example

examples/worker/offscreenCanvas.js

@@ -18,6 +18,8 @@ function init( offscreen, width, height, pixelRatio ) {
 
 	clock = new THREE.Clock();
 
+	// we don't use ImageLoader since it has a DOM dependecy (HTML5 image element)
+
 	var loader = new THREE.ImageBitmapLoader();
 
 	loader.load( '../textures/crate.gif', function ( imageBitmap ) {