Add an array of integers for use as per-screen cursor private keys.

Replaces the use of the screen pointer itself as the key, which was
nice but won't work now that an array index is being stored.

include/cursor.h

@@ -65,6 +65,8 @@ typedef struct _Cursor *CursorPtr;
 typedef struct _CursorMetric *CursorMetricPtr;
 
 extern CursorPtr rootCursor;
+extern int cursorScreenDevPriv[MAXSCREENS];
+#define CursorScreenKey(pScreen) (cursorScreenDevPriv + (pScreen)->myNum)
 
 extern int FreeCursor(
     pointer /*pCurs*/,