Limit lossless refresh update to safe size

We don't want to waste bandwidth on the lossless refresh if we might
need that bandwidth for a normal update. Try to estimate how much
data we can safely send without interfering.
diff --git a/common/rfb/Congestion.h b/common/rfb/Congestion.h
index fd57c22..d293512 100644
--- a/common/rfb/Congestion.h
+++ b/common/rfb/Congestion.h
@@ -1,4 +1,4 @@
-/* Copyright 2009-2015 Pierre Ossman for Cendio AB
+/* Copyright 2009-2018 Pierre Ossman for Cendio AB
  * 
  * This is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -47,6 +47,10 @@
     // longer be congested.
     int getUncongestedETA();
 
+    // getBandwidth() returns the current bandwidth estimation in bytes
+    // per second.
+    size_t getBandwidth();
+
     // debugTrace() writes the current congestion window, as well as the
     // congestion window of the underlying TCP layer, to the specified
     // file
@@ -68,6 +72,8 @@
     unsigned congWindow;
     bool inSlowStart;
 
+    unsigned safeBaseRTT;
+
     struct RTTInfo {
       struct timeval tv;
       unsigned pos;