This allows to flush an async MIDI port's ringbuffer at cycle-start,
to makes its data available as "input/source" during process(), while
collecting data in the background for the next cycle.
This facilitates virtual MIDI Ports for eg. Control Surface Pads:
output from a surface, input to Ardour channels.
The shadow port is a secondary port that can be used to allow others to get a copy of part, all or none of the MIDI
stream received by the owner (input) port.
Increased the size of the FIFO that acts as the intermediate between writers and the MidiBuffer. Changed
implementation of ::write() to notice if MidiBuffer::push_back() fails, and then just leave data queued
for subsequent calls to ::flush_output_fifo().
Note: the logic here will be broken by invalid events/data, which ALSO cause MidiBuffer::push_back() to
return false. That needs fixing
This is not used anywhere in Evoral and is just a wrapper around the PBD
RingBuffer anyway. Towards a (once again?) independently buildable/testable
Evoral and fewer cross-dependencies.
