WebGLProperties: Made get() more WeakMap-ish.

src/renderers/webgl/WebGLProperties.js

@@ -8,16 +8,13 @@ function WebGLProperties() {
 
 	function get( object ) {
 
-		var map = properties.get( object );
+		if ( properties.has( object ) === false ) {
 
-		if ( map === undefined ) {
-
-			map = {};
-			properties.set( object, map );
+			properties.set( object, {} );
 
 		}
 
-		return map;
+		return properties.get( object );
 
 	}