Fix rejected WAVEFORMATEX update for OutputDeviceNodeWasapi

We recently found out that in some cases, particularly on a macbook running windows / bootcamp and default audio settings where it tries to use a quad stereo setup, that the internal summing buffers can become out of sync with the actual AudioRenderClientImpl's configuration.

Solution is to to manually call `Node::setupProcessWithSumming()` after updating the channel count during initialize().  AFAIK, this can't really do any harm, since at most it will try to resize the `audio::BufferDynamic` twice, and the second time around won't do anything.