ui/sdl: Consider scaling in mouse event handling

ui/sdl: Consider scaling in mouse event handling

When using sdl display backend, if the window is scaled, incorrect mouse
positions will be reported since scaling is not properly handled. Fix it
by transforming the positions from window coordinate to guest buffer
coordinate.

Signed-off-by: Weifeng Liu <weifeng.liu@intel.com>
Message-ID: <20250511073337.876650-6-weifeng.liu.z@gmail.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
(cherry picked from commit 30aa105640b0a2a541744b6584d57c9a4b86debd)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>