Non-client scrollbars (SB_HORZ and SB_VERT) don't have their client origin at
(0,0) because they can have non-client borders. It is necessary to offset
by this origin just as it is done for LBUTTONDOWN.
Signed-off-by: Gabriel Ivăncescu <gabrielopcode@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>