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/DesktopWindow.cxx b/vncviewer/DesktopWindow.cxx
index 190b39c..acb0a72 100644
--- a/vncviewer/DesktopWindow.cxx
+++ b/vncviewer/DesktopWindow.cxx
@@ -266,9 +266,9 @@
 
 void DesktopWindow::setCursor(int width, int height,
                               const rfb::Point& hotspot,
-                              void* data, void* mask)
+                              const rdr::U8* data)
 {
-  viewport->setCursor(width, height, hotspot, data, mask);
+  viewport->setCursor(width, height, hotspot, data);
 }