Bugfixes for MIDI Clock Slave

Fixed a couple of bugs so now it might
be fairly usable:

* Song Position works now
* no delay between keyboard and audio for larger period sizes
* keyboards which send MIDI Clock all the time work.

Note that the transport speed might get wobbly,
if the period size is greater than one pulse per quarter note
in frames. eg 1 ppqn @ 120BpM and 44.1 kHz = about 920 frames, so your period size should be at least 512 (lower = better). This is pretty much of a hard limit,
since ardour can only adjust its speed once every period.

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