sdl2: fix mouse grab

When qemu mouse mode changes from relative to absolute
we must turn off sdl relative mouse mode too.

Fixes: https://bugs.launchpad.net/qemu/+bug/1703795
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20180202120803.11501-1-kraxel@redhat.com>

ui/sdl2.c

@@ -249,6 +249,7 @@ static void sdl_mouse_mode_change(Notifier *notify, void *data)
     if (qemu_input_is_absolute()) {
         if (!absolute_enabled) {
             absolute_enabled = 1;
+            SDL_SetRelativeMouseMode(SDL_FALSE);
             absolute_mouse_grab(&sdl2_console[0]);
         }
     } else if (absolute_enabled) {