Stop using fences for pixel format switch

This method was only really useful when continuous updates was active,
so let's rely on a single extension rather than two.
diff --git a/vncviewer/CConn.h b/vncviewer/CConn.h
index dd4ae89..5988a94 100644
--- a/vncviewer/CConn.h
+++ b/vncviewer/CConn.h
@@ -74,6 +74,8 @@
 
   void fence(rdr::U32 flags, unsigned len, const char data[]);
 
+  void endOfContinuousUpdates();
+
   void setLEDState(unsigned int state);
 
 private:
@@ -114,8 +116,6 @@
   bool continuousUpdates;
 
   bool forceNonincremental;
-
-  bool supportsSyncFence;
 };
 
 #endif