source: rework volume handling

- drop the 'virtual_' prefix from s->virtual_volume since we don't
  distuingish between reference and real volumes for sources

- introduce an accuracy for source volumes: if the hardware can control
  the volume "close enough" don't necessarily adjust the rest in
  software unless it is beyond a certain threshold. This should save a
  little bit of CPU at the expensive of a bit of accuracy in volume
  handling.

- other minor cleanups

src/pulsecore/source.h

@@ -79,7 +79,7 @@ struct pa_source {
     pa_volume_t base_volume; /* shall be constant */
     unsigned n_volume_steps; /* shall be constant */
 
-    pa_cvolume virtual_volume, soft_volume;
+    pa_cvolume volume, soft_volume;
     pa_bool_t muted:1;
 
     pa_bool_t refresh_volume:1;