decoder/mpcdec: merge duplicate code

src/decoder/plugins/MpcdecDecoderPlugin.cxx

@@ -137,6 +137,15 @@ mpc_to_mpd_buffer(MpcdecSampleTraits::pointer_type dest,
 		*dest++ = mpc_to_mpd_sample(*src++);
 }
 
+static constexpr ReplayGainTuple
+ImportMpcdecReplayGain(mpc_uint16_t gain, mpc_uint16_t peak) noexcept
+{
+	auto t = ReplayGainTuple::Undefined();
+	t.gain = MPC_OLD_GAIN_REF - (gain  / 256.);
+	t.peak = pow(10, peak / 256. / 20) / 32767;
+	return t;
+}
+
 static void
 mpcdec_decode(DecoderClient &client, InputStream &is)
 {
@@ -169,10 +178,8 @@ mpcdec_decode(DecoderClient &client, InputStream &is)
 
 	ReplayGainInfo rgi;
 	rgi.Clear();
-	rgi.album.gain = MPC_OLD_GAIN_REF  - (info.gain_album  / 256.);
-	rgi.album.peak = pow(10, info.peak_album / 256. / 20) / 32767;
-	rgi.track.gain = MPC_OLD_GAIN_REF  - (info.gain_title  / 256.);
-	rgi.track.peak = pow(10, info.peak_title / 256. / 20) / 32767;
+	rgi.album = ImportMpcdecReplayGain(info.gain_album, info.peak_album);
+	rgi.track = ImportMpcdecReplayGain(info.gain_title, info.peak_title);
 
 	client.SubmitReplayGain(&rgi);