output/Source: release the mutex while the filter runs

The filter can take some time to finish, even more so on a weak
machine with a saturated CPU.  By not holding the mutex during that
time, we can reduce PlayerThread latency a lot, because that thread
needs to synchronize a lot with all outputs.

src/output/OutputThread.cxx

@@ -264,7 +264,7 @@ AudioOutput::WaitForDelay()
 bool
 AudioOutput::FillSourceOrClose()
 try {
-	return source.Fill();
+	return source.Fill(mutex);
 } catch (const std::runtime_error &e) {
 	FormatError(e, "Failed to filter for output \"%s\" [%s]",
 		    name, plugin.name);