gst: add rate control to the sink

Track the elapsed time between buffers and try to keep the buffer fill
level constant by changing the rate of the stream.

See #4374

src/gst/gstpipewiresink.h

@@ -50,6 +50,8 @@ struct _GstPipeWireSink {
 
   /* video state */
   gboolean negotiated;
+  gboolean rate_match;
+  gint rate;
 
   GstPipeWireSinkMode mode;
 };