decoder/Client: new interface which wraps struct Decoder

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

test/run_decoder.cxx

@@ -42,7 +42,7 @@ try {
 		return EXIT_FAILURE;
 	}
 
-	Decoder decoder;
+	FakeDecoder decoder;
 	const char *const decoder_name = argv[1];
 	const char *const uri = argv[2];