Document why we need to zero-initialize svt_config

src/codec_svt.c

diff --git a/src/codec_svt.c b/src/codec_svt.c
index a28d2c3..4687e2b 100644
--- a/src/codec_svt.c
+++ b/src/codec_svt.c
@@ -65,6 +65,8 @@
         EbSvtAv1EncConfiguration * svt_config = avifAlloc(sizeof(EbSvtAv1EncConfiguration));
         if (!svt_config)
             return AVIF_RESULT_UNKNOWN_ERROR;
+        // Zero-initialize svt_config because svt_av1_enc_init_handle() does not set many fields of svt_config.
+        // See https://gitlab.com/AOMediaCodec/SVT-AV1/-/issues/1697.
         memset(svt_config, 0, sizeof(EbSvtAv1EncConfiguration));
         codec->internal->svt_config = svt_config;