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.h b/common/rfb/ClientParams.h
index 63d4e19..f7a7044 100644
--- a/common/rfb/ClientParams.h
+++ b/common/rfb/ClientParams.h
@@ -87,6 +87,7 @@
     // Wrappers to check for functionality rather than specific
     // encodings
     bool supportsLocalCursor() const;
+    bool supportsDesktopSize() const;
     bool supportsLEDState() const;
     bool supportsFence() const;
     bool supportsContinuousUpdates() const;