QQuickWindow::setColor() should use update() rather than dirtyItem()

dirtyItem() will trigger maybeUpdate in the renderloop which may
decide to skip the render pass if nothing changed in the scene
graph. Use update() instead which forces the rendering to go through
regardless.

Change-Id: I4730bb0cd79d58fa21970a847cc1720060cb14a8
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>