Switch from XMLHttpRequest.DONE to 4 because IE8

src/Tools/babylon.tools.ts

@@ -394,7 +394,7 @@
                 request.onprogress = progressCallBack;
 
                 request.onreadystatechange = () => {
-                    if (request.readyState === XMLHttpRequest.DONE) {
+                    if (request.readyState === 4) {
                         request.onreadystatechange = null;//some browsers have issues where onreadystatechange can be called multiple times with the same value
 
                         if (request.status >= 200 && request.status < 300) {