Do not assume window is defined

It is notably not the case in web workers.

src/ol/index.js

@@ -41,7 +41,7 @@ let WEBGL_EXTENSIONS; // value is set below
 let HAS_WEBGL = false;
 
 
-if ('WebGLRenderingContext' in window) {
+if (typeof window !== 'undefined' && 'WebGLRenderingContext' in window) {
   try {
     const canvas = /** @type {HTMLCanvasElement} */
         (document.createElement('CANVAS'));