ui/sdl2: Grab Alt+Tab also in fullscreen mode

ui/sdl2: Grab Alt+Tab also in fullscreen mode

By default, SDL grabs Alt+Tab only in non-fullscreen mode. This causes Alt+Tab
to switch tasks on the host rather than in the VM in fullscreen mode while it
switches tasks in non-fullscreen mode in the VM. Fix this confusing behavior
by grabbing Alt+Tab in fullscreen mode, always causing tasks to be switched in
the VM.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20230417192139.43263-2-shentey@gmail.com>
(cherry picked from commit efc00a37090eced53bff8b42d26991252aaacc44)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>