pcm/Volume: Open() returns output sample format

Prepare for a new mode which may convert to a different sample format
when applying volume, to reduce dithering.

src/filter/plugins/ReplayGainFilterPlugin.cxx

@@ -77,7 +77,7 @@ public:
 		 mixer(_mixer), base(_base) {
 		info.Clear();
 
-		pv.Open(out_audio_format.format);
+		out_audio_format.format = pv.Open(out_audio_format.format);
 	}
 
 	void SetInfo(const ReplayGainInfo *_info) {

src/filter/plugins/VolumeFilterPlugin.cxx

@@ -30,7 +30,7 @@ class VolumeFilter final : public Filter {
 public:
 	explicit VolumeFilter(const AudioFormat &audio_format)
 		:Filter(audio_format) {
-		pv.Open(out_audio_format.format);
+		out_audio_format.format = pv.Open(out_audio_format.format);
 	}
 
 	unsigned GetVolume() const noexcept {

src/pcm/Volume.cxx

@@ -96,7 +96,7 @@ pcm_volume_change_float(float *dest, const float *src, size_t n,
 		dest[i] = src[i] * volume;
 }
 
-void
+SampleFormat
 PcmVolume::Open(SampleFormat _format)
 {
 	assert(format == SampleFormat::UNDEFINED);
@@ -118,7 +118,7 @@ PcmVolume::Open(SampleFormat _format)
 		break;
 	}
 
-	format = _format;
+	return format = _format;
 }
 
 ConstBuffer<void>

src/pcm/Volume.hxx

@@ -94,9 +94,10 @@ public:
 	 *
 	 * Throws on error.
 	 *
-	 * @param format the sample format
+	 * @param format the input sample format
+	 * @return the output sample format
 	 */
-	void Open(SampleFormat format);
+	SampleFormat Open(SampleFormat format);
 
 	/**
 	 * Closes the object.  After that, you may call Open() again.