decoder/Thread: don't scan for replay gain tags in PCM streams

This disables a long delay for playing songs from the cdio_paranoia
input plugin if ReplayGain is enabled.

src/decoder/Thread.cxx

@@ -36,6 +36,7 @@
 #include "util/RuntimeError.hxx"
 #include "util/Domain.hxx"
 #include "util/ScopeExit.hxx"
+#include "util/StringCompare.hxx"
 #include "thread/Name.hxx"
 #include "tag/ApeReplayGain.hxx"
 #include "Log.hxx"
@@ -265,6 +266,13 @@ MaybeLoadReplayGain(DecoderBridge &bridge, InputStream &is)
 		/* ReplayGain is disabled */
 		return;
 
+	if (is.HasMimeType() &&
+	    StringStartsWith(is.GetMimeType(), "audio/x-mpd-"))
+		/* skip for (virtual) files (e.g. from the
+		   cdio_paranoia input plugin) which cannot possibly
+		   contain tags */
+		return;
+
 	LoadReplayGain(bridge, is);
 }