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/Viewport.h b/vncviewer/Viewport.h
index 0967fcb..6f0710d 100644
--- a/vncviewer/Viewport.h
+++ b/vncviewer/Viewport.h
@@ -45,7 +45,7 @@
 
   // New image for the locally rendered cursor
   void setCursor(int width, int height, const rfb::Point& hotspot,
-                 void* data, void* mask);
+                 const rdr::U8* data);
 
   void draw(Surface* dst);