replaygain: R128_ALBUM_GAIN tag format (Q7.8)

Store `R129_ALBUM_GAIN`'s value as a Q7.8 fixed-point number as required by IETF
RFC 7845 "Ogg Encapsulation for the Opus Audio Codec" 5.2.1. Tag Definitions.

beetsplug/replaygain.py

@@ -83,6 +83,14 @@ def lufs_to_db(db):
     return db + 107
 
 
+def gain_to_q78(gain):
+    """Generate a Q7.8 fixed-point gain.
+
+    Convert a `Gain` object to the representation used e.g. in OPUS tags.
+    """
+    return int(round(gain.gain * pow(2, 8)))
+
+
 # Backend base and plumbing classes.
 
 # gain: in LU to reference level
@@ -1191,25 +1199,23 @@ def store_track_gain(self, item, track_gain):
         item.rg_track_gain = track_gain.gain
         item.rg_track_peak = track_gain.peak
         item.store()
-
         self._log.debug(u'applied track gain {0}, peak {1}',
                         item.rg_track_gain, item.rg_track_peak)
 
-    def store_track_r128_gain(self, item, track_gain):
-        item.r128_track_gain = int(round(track_gain.gain * pow(2, 8)))
-        item.store()
-
-        self._log.debug(u'applied r128 track gain {0}', item.r128_track_gain)
-
     def store_album_gain(self, item, album_gain):
         item.rg_album_gain = album_gain.gain
         item.rg_album_peak = album_gain.peak
         item.store()
         self._log.debug(u'applied album gain {0}, peak {1}',
                         item.rg_album_gain, item.rg_album_peak)
 
+    def store_track_r128_gain(self, item, track_gain):
+        item.r128_track_gain = gain_to_q78(track_gain)
+        item.store()
+        self._log.debug(u'applied r128 track gain {0}', item.r128_track_gain)
+
     def store_album_r128_gain(self, item, album_gain):
-        item.r128_album_gain = album_gain.gain
+        item.r128_album_gain = gain_to_q78(album_gain)
         item.store()
         self._log.debug(u'applied r128 album gain {0}',
                         item.r128_album_gain)