ui/gtk: force realization of drawing area

ui/gtk: force realization of drawing area

Fixes the GL context creation from a widget that isn't yet realized (in
a hidden tab for example).

Resolves:
https://gitlab.com/qemu-project/qemu/-/issues/1727

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Antonio Caggiano <quic_acaggian@quicinc.com>
Message-Id: <20231017111642.1155545-1-marcandre.lureau@redhat.com>
(cherry picked from commit 565f85a9c293818a91a3d3414311303de7e00cec)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>