Remove alpha-related fields from visual structure to retain binary

compatibilty. Applications using ARGB visuals will need to use Render
    or other mechanisms to compute pixel values instead of AllocColor

include/scrnintstr.h

@@ -75,10 +75,6 @@ typedef struct _Visual {
 				 * it may have more or fewer */
     unsigned long	redMask, greenMask, blueMask;
     int			offsetRed, offsetGreen, offsetBlue;
-#ifdef COMPOSITE
-    unsigned long	alphaMask;
-    int			offsetAlpha;
-#endif
   } VisualRec;
 
 typedef struct _Depth {