gst: keep track of clock time

Invalidate the clock when the stream is destroyed and let the new
clock take the time of previous clock when no timing info is
available.

src/gst/gstpipewiresrc.h

@@ -82,6 +82,7 @@ struct _GstPipeWireSrc {
   GstPipeWirePool *pool;
   GQueue queue;
   GstClock *clock;
+  GstClockTime last_time;
 };
 
 struct _GstPipeWireSrcClass {