Dividing an unsigned int by 2 is different from dividing an int by 2

This division was the cause for vlc(Win32) displaying a black screen
when zooming exceeded the display size (alt-'o')

Weirdly, there was no problem for Linux !!?? and also no regression :)
(cherry picked from commit 30efd37a)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>