Add crude congestion window debug trace

Allows us to compare our computed congestion window with the
underlying one used by the TCP layer.
diff --git a/common/rfb/Congestion.h b/common/rfb/Congestion.h
index 5feea65..fd57c22 100644
--- a/common/rfb/Congestion.h
+++ b/common/rfb/Congestion.h
@@ -47,6 +47,11 @@
     // longer be congested.
     int getUncongestedETA();
 
+    // debugTrace() writes the current congestion window, as well as the
+    // congestion window of the underlying TCP layer, to the specified
+    // file
+    void debugTrace(const char* filename, int fd);
+
   protected:
     unsigned getExtraBuffer();
     unsigned getInFlight();