screencasting: Resume streams after buffer renegotiation

When we start renegotiation the stream is pasused. This is not restarted
when the format has been renegotiated.

This occurred when a streaming window was resized

src/plugins/screencast/screencaststream.cpp

@@ -207,6 +207,7 @@ void ScreenCastStream::onStreamParamChanged(uint32_t id, const struct spa_pod *f
 
     qCDebug(KWIN_SCREENCAST) << "Stream format found, defining buffers";
     newStreamParams();
+    m_streaming = true;
 }
 
 void ScreenCastStream::onStreamAddBuffer(pw_buffer *buffer)