Fix bad signed/unsigned comparisons

Either by casting, or switching to a more appropriate type
for the variable.
diff --git a/common/rfb/EncodeManager.cxx b/common/rfb/EncodeManager.cxx
index e632387..4e133d3 100644
--- a/common/rfb/EncodeManager.cxx
+++ b/common/rfb/EncodeManager.cxx
@@ -152,7 +152,7 @@
 
 void EncodeManager::logStats()
 {
-  int i, j;
+  size_t i, j;
 
   unsigned rects;
   unsigned long long pixels, bytes, equivalent;
@@ -603,7 +603,7 @@
   Encoder *encoder;
 
   struct RectInfo info;
-  int divisor, maxColours;
+  unsigned int divisor, maxColours;
 
   bool useRLE;
   EncoderType type;