Decouple check for existence from building draw lists.

We need to know if samples, context switches and messages are present to be
able to correctly calculate thread height. However, if the thread is not
visible, it is not necessary to provide a list of items to draw.

server/TracyTimelineItemThread.hpp

@@ -54,6 +54,9 @@ private:
     std::vector<TimelineDraw> m_draw;
     std::vector<MessagesDraw> m_msgDraw;
     int m_depth;
+    bool m_hasCtxSwitch;
+    bool m_hasSamples;
+    bool m_hasMessages;
 };
 
 }