decoder/Client: new interface which wraps struct Decoder

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

src/decoder/plugins/FfmpegIo.cxx

@@ -38,7 +38,7 @@ AvioStream::~AvioStream()
 inline int
 AvioStream::Read(void *dest, int size)
 {
-	return decoder_read(decoder, input, dest, size);
+	return decoder_read(client, input, dest, size);
 }
 
 inline int64_t