pleorasrc: set caps before _create to avoid bad negotiation

Caps must be fixated before _create, otherwise initial caps is
width=1,height=1 which cause some sinks to create incorrect window
sizes.

sys/pleora/gstpleorasrc.h

@@ -45,6 +45,7 @@ struct _GstPleoraSrc
   PvPipeline *pipeline;
   PvDevice *device;
   PvStream *stream;
+  PvBuffer *pvbuffer;
 
   /* properties */
   gchar *device_id;