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.

dix/globals.c

@@ -142,6 +142,8 @@ CursorPtr rootCursor;
 Bool blackRoot=FALSE;
 Bool whiteRoot=FALSE;
 
+_X_EXPORT int cursorScreenDevPriv[MAXSCREENS];
+
 _X_EXPORT TimeStamp currentTime;
 _X_EXPORT TimeStamp lastDeviceEventTime;