Setting intra_period_length to 0 doesn't work

We are using SVT-AV1 v1.5.0 in libavif. This issue was originally reported during the review of libavif pull request https://github.com/AOMediaCodec/libavif/pull/1399. @hassount @1480c1 FYI.

We would like to set intra_period_length to 0 so that all frames are encoded as key frames. But when we set intra_period_length to 0 in our code (https://github.com/AOMediaCodec/libavif/blob/main/src/codec_svt.c), it behaves the same way as though we never set it, i.e., the first frame is a key frame and subsequent frames are not.

Right now, to achieve "all intra" encoding, we set force_key_frames to TRUE and set the pic_type field of EbBufferHeaderType to EB_AV1_KEY_PICTURE for every frame.