This mostly reverts commit 6e89e61b89.

MIDI FX plugins are allowed to have audio channels, even if they only
produce silence. If a plugin requests a particular bus/channel layout,
the host should respect this request, or attempt to update the layout.
The host should fill all channels that the plugin expects to be filled.

The old commit also meant that we weren't correctly supplying the host
sample rate to MIDI FX plugins.