sdl2: Do not leave grab when fullscreen

Prevents displaying of a doubled mouse pointer when moving the pointer
to the screen edges when fullscreen.

Signed-off-by: Jindrich Makovicka <makovick@gmail.com>
Message-Id: <20171112193032.9724-8-makovick@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
