Change cursor API to use RGBA data

This will allow us to use better formats that preserve the entire
alpha channel.
diff --git a/common/rfb/CMsgHandler.h b/common/rfb/CMsgHandler.h
index 7d2cdc2..2686712 100644
--- a/common/rfb/CMsgHandler.h
+++ b/common/rfb/CMsgHandler.h
@@ -50,7 +50,7 @@
                                         int w, int h,
                                         const ScreenSet& layout);
     virtual void setCursor(int width, int height, const Point& hotspot,
-                           void* data, void* mask) = 0;
+                           const rdr::U8* data) = 0;
     virtual void setPixelFormat(const PixelFormat& pf);
     virtual void setName(const char* name);
     virtual void fence(rdr::U32 flags, unsigned len, const char data[]);