further mouse relative motion fix for os/2 (thanks Andrey Vasilkin)

src/video/os2/SDL_os2mouse.c

@@ -122,11 +122,9 @@ static void OS2_WarpMouse(SDL_Window * window, int x, int y)
     POINTL      pointl;
 
     pointl.x = x;
-    pointl.y = window->h - y;
+    pointl.y = window->h - y - 1;
     WinMapWindowPoints(pWinData->hwnd, HWND_DESKTOP, &pointl, 1);
-/*  pWinData->lSkipWMMouseMove++; ???*/
     WinSetPointerPos(HWND_DESKTOP, pointl.x, pointl.y);
-
     SDL_SendMouseMotion(window, SDL_GetMouse()->mouseID, 0, x, y);
 }