Use the new UMP alsa sequencer API to make it produce UMP packets.
Set the alsa sequencer to MIDI2.0, which will make it convert all
messages to MIDI-2.0 UMP automatically. We can copy this straight into
the control buffers.
This also solves some problems with large sysex messages that are now
nicely split into chunks with UMP.
The default kernel pool size on the input is 200 cells. A cell is
about 28 bytes long so the maximum message that can be received in one
go is about 5600 bytes. This causes problems when using amidi to upload
larger sysex messages because they simply can't be received by the
sequencer.
It if however possible to increase this limit with the set_client_pool()
function. Increase the pool size to at least the quantum_limit * 2.
This ensures we can receive and send at least 2 quantums of raw data,
which should be a fairly long sysex message.
Make a min and max value for the pool size. There is an upper limit of
2000 in the kernel but make this configurable and clamp the final
pool size to the min/max.
Make the MAX_EVENT_SIZE 256, because this is how the sequencer seems to
splits the input data as well and it results in less wasted space in the
output buffer.
See #4005
The alsa sequencer rate matching was not actually working correctly.
It would compare the previous queue time with the current time and
compare that to the quantum. This would include uncorrected errors from
jitter and would result in the timeouts being scaled in the wrong
direction forever.
Instead, calculate an ideal queue time and compare our current queue
time against that. We then use the correction to scale the timeout or
the next queue time prediction.
Also use the predicted time as the base time for the event timestamps.
this results in less jitter.
Fixes#3657
