Pass value object attributes by reference (instead of pointer)

This underlines that no reference inside the setter is taken,
and that these attributes are non-nullable.

Also removes a couple of superfluous attribute macros.

binding-mri/window-binding.cpp

@@ -34,7 +34,7 @@ RB_METHOD(windowInitialize)
 
 	w->initDynAttribs();
 
-	wrapProperty(self, w->getCursorRect(), "cursor_rect", RectType);
+	wrapProperty(self, &w->getCursorRect(), "cursor_rect", RectType);
 
 	return self;
 }