[pulseaudio-discuss] [WIP] Passthrough support

[pl bossart]

> I pushed your change along with a fix for the assert you saw to my tree
> as well.

I tested the latest batch of patches with AC3/SPDIF this time. Works
well in general, however I found some interesting points:
- in Rhythmbox, the passthrough mode is only enabled if the first file
of playlist is compressed. If you start with PCM/WAV, it always uses
the PCM mode.
- in Totem, the passthrough mode works when the visual effects are
disabled. I see a nice DD on my receiver...However the playback stops
if you play with the volume slider. I get a pop-up error window saying
'an error occurred: pa_stream_set_sink_input_volume(): invalid
argument'

On a related note, maybe we want to change the approach to volume
control. Depending on the sink, there might be ways of using
side/in-band channels to send volume control information to the
decoder. It's going to be protocol-specific. So maybe we need to
provide the client with an information coming from the sink on whether
volume control is actually supported or not.
-Piere