SVT-AV1 does not support requesting keyframes at arbitrary points
by setting pic_type to EB_AV1_KEY_PICTURE. So set force_key_frames
to 1 only when gop_size == 1.

Please see the comments in
https://gitlab.com/AOMediaCodec/SVT-AV1/-/issues/2076 for a bit more
details.
Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>