Abstract sending cursor and resizing the desktop

Avoid having the callers need to know about the different variants
of these functions and instead have the writer pick the most appropriate
extension.
diff --git a/common/rfb/ClientParams.cxx b/common/rfb/ClientParams.cxx
index 7838407..2f8783b 100644
--- a/common/rfb/ClientParams.cxx
+++ b/common/rfb/ClientParams.cxx
@@ -147,6 +147,15 @@
   return false;
 }
 
+bool ClientParams::supportsDesktopSize() const
+{
+  if (supportsEncoding(pseudoEncodingExtendedDesktopSize))
+    return true;
+  if (supportsEncoding(pseudoEncodingDesktopSize))
+    return true;
+  return false;
+}
+
 bool ClientParams::supportsLEDState() const
 {
   if (supportsEncoding(pseudoEncodingLEDState))