We need to specify SDL_WINDOW_OPENGL if we want to create an OpenGL context on it, i.e. when using '-device virtio-gpu-pci,virgl=on'

Signed-off-by: Jan Henrik Weinstock <jan.weinstock@rwth-aachen.de>
Message-id: b2ba98b3-2975-0d4d-1c56-f659923c714d@rwth-aachen.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>