xwin: In multiwindow mode, look up the HWND for the parent window

Rather than only looking at the foreground window to see if it matches
the WM_TRANSIENT_FOR window XID, lookup that XID and fetch the HWND from
the window privates.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/winmultiwindowwindow.c

@@ -523,9 +523,13 @@ winCreateWindowsWindow(WindowPtr pWin)
 
     if (winMultiWindowGetTransientFor(pWin, &daddyId)) {
         if (daddyId) {
-            hFore = GetForegroundWindow();
-            if (hFore && (daddyId != (Window) (INT_PTR) GetProp(hFore, WIN_WID_PROP)))
-                hFore = NULL;
+            WindowPtr pParent;
+            int res = dixLookupWindow(&pParent, daddyId, serverClient, DixReadAccess);
+            if (res == Success)
+                {
+                    winPrivWinPtr pParentPriv = winGetWindowPriv(pParent);
+                    hFore = pParentPriv->hWnd;
+                }
         }
     }
     else {