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/gst/gstpinospool.c

@@ -75,6 +75,7 @@ acquire_buffer (GstBufferPool * pool, GstBuffer ** buffer,
 
   GST_OBJECT_LOCK (pool);
   while (p->available.length == 0) {
+    GST_WARNING ("queue empty");
     g_cond_wait (&p->cond, GST_OBJECT_GET_LOCK (pool));
   }
   *buffer = g_queue_pop_head (&p->available);