sink, source: Add an avoid-processing mode

This generalises the avoid-resampling concept (don't resample for any
rate above the default/alternate sample rate) to include channel count
and sample format as well. The rationale for this is that users who wish
to send out their data untouched by processing in PulseAudio can do so.

In addition to this, there are opportunities for certain hardware (such
as systems with a DSP connected to a codec) to offload processing to the
DSP (providing potential cost savings).

Finally, this also enables modules that might be able to perform
transformations of (ANY -> sink format), and this allows us to implement
such transformations.

src/pulsecore/core.h

@@ -221,6 +221,7 @@ struct pa_core {
     bool running_as_daemon:1;
     bool realtime_scheduling:1;
     bool avoid_resampling:1;
+    bool avoid_processing:1;
     bool disable_remixing:1;
     bool remixing_use_all_sink_channels:1;
     bool remixing_produce_lfe:1;