Improve upload

Only send a buffer when we have received a NEED_DATA message.
Add a signal to pulla buffer from the sink. Restructure the sink to use
a queue like the source and only push a buffer when we can.
Improve SpaData. Offset and size should be between 0 and maxsize, make
sure we clamp correctly when needed.
node_process_output completes the processing of the output after
receiving HAVE_OUTPUT for async elements. It instructs the node that
it now can produce more output.

pinos/client/stream.h

@@ -91,6 +91,8 @@ struct _PinosStream {
   PINOS_SIGNAL (new_buffer,    (PinosListener *listener,
                                 PinosStream   *stream,
                                 uint32_t       id));
+  PINOS_SIGNAL (need_buffer,   (PinosListener *listener,
+                                PinosStream   *stream));
 };
 
 PinosStream *    pinos_stream_new               (PinosContext    *context,