ui/sdl2: disable SDL_HINT_GRAB_KEYBOARD on Windows

ui/sdl2: disable SDL_HINT_GRAB_KEYBOARD on Windows

Windows sends an extra left control key up/down input event for
every right alt key up/down input event for keyboards with
international layout. Since commit 830473455f ("ui/sdl2: fix
handling of AltGr key on Windows") QEMU uses a Windows low level
keyboard hook procedure to reliably filter out the special left
control key and to grab the keyboard on Windows.

The SDL2 version 2.0.16 introduced its own Windows low level
keyboard hook procedure to grab the keyboard. Windows calls this
callback before the QEMU keyboard hook procedure. This disables
the special left control key filter when the keyboard is grabbed.

To fix the problem, disable the SDL2 Windows low level keyboard
hook procedure.

Reported-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230418062823.5683-1-vr_qemu@t-online.de>