pcm_volume: added constant PCM_VOLUME_1

It may be desirable to change the range of integer volume levels
(e.g. to 1024, which may utilize shifts instead of expensive integer
divisions).  Introduce the constant PCM_VOLUME_1 which describes the
integer value for "100% volume".  This is currently 1000.

src/pcm_utils.h

@@ -27,6 +27,11 @@
 
 struct audio_format;
 
+enum {
+	/** this value means "100% volume" */
+	PCM_VOLUME_1 = 1000,
+};
+
 struct pcm_convert_state {
 	struct pcm_resample_state resample;
 
@@ -43,7 +48,7 @@ struct pcm_convert_state {
 static inline int
 pcm_float_to_volume(float volume)
 {
-	return volume * 1000.0 + 0.5;
+	return volume * PCM_VOLUME_1 + 0.5;
 }
 
 void pcm_volume(char *buffer, int bufferSize,