gif decoder reuses buffer without flagging it

[delusional]

According to the get_buffer2 documentation the codec is supposed to signal that it'll reuse the returned buffer with the flag AV_GET_BUFFER_FLAG_REF[1].

The gif decoder doesn't set this flag

libavcodec/gifdec.c:492 if ((ret = ff_get_buffer(avctx, s->frame, 0)) < 0)

yet attempts to reuse the buffer until the next keyframe

libavcodec/gifdec.c:508 if ((ret = ff_reget_buffer(avctx, s->frame, 0)) < 0)

This leads to a bug in libav users that rely on get_buffer2 not being reused like gstreamer[2].

[1]: https://ffmpeg.org/doxygen/6.0/structAVCodecContext.html#aef79333a4c6abf1628c55d75ec82bede
[2]: ​https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2792

[Elon Musk]

Fixed in 891167dee584b7c7760fe9a54bb65aec9bca0a56