Change cursor API to use RGBA data

This will allow us to use better formats that preserve the entire
alpha channel.
diff --git a/vncviewer/CConn.h b/vncviewer/CConn.h
index c934f3d..d6dd4a7 100644
--- a/vncviewer/CConn.h
+++ b/vncviewer/CConn.h
@@ -66,7 +66,7 @@
   void dataRect(const rfb::Rect& r, int encoding);
 
   void setCursor(int width, int height, const rfb::Point& hotspot,
-                 void* data, void* mask);
+                 const rdr::U8* data);
 
   void fence(rdr::U32 flags, unsigned len, const char data[]);