Merge branch 'master' of https://github.com/BabylonJS/Babylon.js

Babylon/babylon.engine.js

@@ -11,7 +11,7 @@
             throw new Error("WebGL not supported");
         }
 
-        if (this._gl === undefined) {
+        if (this._gl === undefined || this._gl === null) {
             throw new Error("WebGL not supported");
         }
 
@@ -937,4 +937,4 @@
             return false;
         }
     };
-})();
+})();