QOpenGLFramebufferObject::blitFramebuffer does not use default FBO if none specified

See summary. The documentation says that if either the source or target buffer is NULL, that "the default framebuffer will be used". However this is not the case; instead, FBO 0 is used. This makes it impossible to use these functions with a QOpenGLWidget, as the widget's QOpenGLFramebufferObject is not available to the user, and there is no way to tell QOpenGLFramebufferObject::blitFramebuffer to use a buffer identified only by its handle.