Add support for MixRamp tags

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

test/run_decoder.c

@@ -142,6 +142,14 @@ decoder_replay_gain(G_GNUC_UNUSED struct decoder *decoder,
 {
 }
 
+void
+decoder_mixramp(G_GNUC_UNUSED struct decoder *decoder,
+		char *mixramp_start, char *mixramp_end)
+{
+	g_free(mixramp_start);
+	g_free(mixramp_end);
+}
+
 int main(int argc, char **argv)
 {
 	GError *error = NULL;