Change cursor API to use RGBA data

This will allow us to use better formats that preserve the entire
alpha channel.
diff --git a/tests/decperf.cxx b/tests/decperf.cxx
index 1fd763e..3b929a4 100644
--- a/tests/decperf.cxx
+++ b/tests/decperf.cxx
@@ -49,7 +49,7 @@
 
   virtual void setDesktopSize(int w, int h);
   virtual void setPixelFormat(const rfb::PixelFormat& pf);
-  virtual void setCursor(int, int, const rfb::Point&, void*, void*);
+  virtual void setCursor(int, int, const rfb::Point&, const rdr::U8*);
   virtual void framebufferUpdateStart();
   virtual void framebufferUpdateEnd();
   virtual void setColourMapEntries(int, int, rdr::U16*);
@@ -94,7 +94,7 @@
   CConnection::setPixelFormat(filePF);
 }
 
-void CConn::setCursor(int, int, const rfb::Point&, void*, void*)
+void CConn::setCursor(int, int, const rfb::Point&, const rdr::U8*)
 {
 }