pcm/Volume: add constant PCM_VOLUME_BITS

src/pcm/PcmVolume.cxx

@@ -37,7 +37,7 @@ pcm_volume_change(typename Traits::pointer_type buffer,
 
 		sample = (sample * volume + pcm_volume_dither() +
 			  PCM_VOLUME_1S / 2)
-			/ PCM_VOLUME_1S;
+			>> PCM_VOLUME_BITS;
 
 		*buffer++ = PcmClamp<F, Traits>(sample);
 	}

src/pcm/PcmVolume.hxx

@@ -27,6 +27,11 @@
 #include <stddef.h>
 
 /**
+ * Number of fractional bits for a fixed-point volume value.
+ */
+static constexpr unsigned PCM_VOLUME_BITS = 10;
+
+/**
  * This value means "100% volume".
  */
 static constexpr unsigned PCM_VOLUME_1 = 1024;