plugins/screencast: Reset last sent timestamp when the stream resumes

Hypothetically, if the stream is paused and restarted in a short burst,
the frame throttling logic can be mis-triggered.

src/plugins/screencast/screencaststream.cpp

@@ -90,6 +90,7 @@ void ScreenCastStream::onStreamStateChanged(pw_stream_state old, pw_stream_state
         break;
     case PW_STREAM_STATE_STREAMING:
         m_streaming = true;
+        m_lastSent = QDateTime();
         m_source->resume();
         break;
     case PW_STREAM_STATE_CONNECTING: