gst: handle latency in the pipewiresrc

Get the delay in the graph and subtract this from the cycle start time
to get the timestamp of the buffer.

Report this latency as well.

Fixes #30

src/gst/gstpipewiresrc.h

@@ -68,6 +68,7 @@ struct _GstPipeWireSrc {
   gboolean eos;
 
   gboolean is_live;
+  int64_t delay;
   GstClockTime min_latency;
   GstClockTime max_latency;