on_update is for pull-model outpput targets to do periodic synchronous
checks (eg, checking that the connection to the actual output device is
still alive and reviving it if necessary)
Output plugins can use either a push model (synchronous) or a pull
model (asynchronous). The ALSA plugin now uses the pull model. This
paves the way for making jack output a simple output plugin rather than
the combined render/output plugin it currently is (for #16) as now
snd_dma works with both models.
This brings the alsa driver in line with the jack render (progress
towards #16), but breaks most of the other drivers (for now: one step at
a time). The idea is that once the pull model is working for at least
one other target, the jack renderer can become just another target like
it should have been in the first place (but I needed to get the pull
model working first, then forgot about it).
Correct state checking is not done yet, but testsound does produce what
seems to be fairly good sound when it starts up correctly (part of the
state checking (or lack thereof), I imagine).
