Add UTF-8 to/from ISO 8859-1 conversion routines

We convert between UTF-8 and ISO 8859-1 (latin 1) in several places
so create some common routines for this.
diff --git a/vncviewer/Viewport.h b/vncviewer/Viewport.h
index c2c9872..8b9b469 100644
--- a/vncviewer/Viewport.h
+++ b/vncviewer/Viewport.h
@@ -114,8 +114,8 @@
 
   bool firstLEDState;
 
-  const char* pendingServerCutText;
-  const char* pendingClientCutText;
+  char* pendingServerCutText;
+  char* pendingClientCutText;
 
   rdr::U32 menuKeySym;
   int menuKeyCode, menuKeyFLTK;