jitterbuffer: return NoPreroll when going from READY to PAUSED

This is useful when upstream is not a live source

Fixes potential issue discussed in:

https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/issues/93#note_388528

gst-plugin-threadshare/src/jitterbuffer/jitterbuffer.rs

@@ -1373,7 +1373,13 @@ impl ElementImpl for JitterBuffer {
             _ => (),
         }
 
-        self.parent_change_state(element, transition)
+        let mut ret = self.parent_change_state(element, transition)?;
+
+        if transition == gst::StateChange::ReadyToPaused {
+            ret = gst::StateChangeSuccess::NoPreroll;
+        }
+
+        Ok(ret)
     }
 }