Replace checking if image has to DataURL with checking if image is instanceof...

Replace checking if image has to DataURL with checking if image is instanceof HTMLCanvasElement in getDataURL() of Texture

src/textures/Texture.js

@@ -125,7 +125,7 @@ Object.assign( Texture.prototype, EventDispatcher.prototype, {
 
 			var canvas;
 
-			if ( image.toDataURL !== undefined ) {
+			if ( image instanceof HTMLCanvasElement ) {
 
 				canvas = image;