Scrollbar tracking fix

[Duane Clark <dclark@akamail.com>]

This fixes a behavior, especially noticeable in listboxes and listviews, 
where clicking below the scrollbar a couple of times (paging down) and 
then clicking in the thumb, causes the list contents to jump up while 
the mouse button is held down.

Changelog:
	Need to initialize tracking value on mousedown.
Index: controls/scroll.c
===================================================================
RCS file: /home/wine/wine/controls/scroll.c,v
retrieving revision 1.60
diff -u -r1.60 scroll.c
--- controls/scroll.c	3 Dec 2002 23:34:54 -0000	1.60
+++ controls/scroll.c	17 Dec 2002 02:34:59 -0000
@@ -1023,6 +1023,9 @@
             SCROLL_TrackingWin = hwnd;
             SCROLL_TrackingBar = nBar;
             SCROLL_TrackingPos = trackThumbPos + lastMousePos - lastClickPos;
+            SCROLL_TrackingVal = SCROLL_GetThumbVal( infoPtr, &rect,
+                                                        vertical,
+                                                        SCROLL_TrackingPos );
 	    if (!SCROLL_MovingThumb)
 		SCROLL_DrawMovingThumb(hdc, &rect, vertical, arrowSize, thumbSize);
         }