Fix dispose (WeakMap.clear is obsolete and should not be used)

src/renderers/webgl/WebGLObjects.js

@@ -41,7 +41,7 @@ function WebGLObjects( gl, geometries, attributes, info ) {
 
 	function dispose() {
 
-		updateMap.clear();
+		updateMap = new WeakMap();
 
 	}