[FFmpeg-cvslog] avcodec/ac3dec: only export matrix encoding and downmix info side data when necessary

ffmpeg | branch: master | James Almer | Wed Jan 15 22:19:54 2025 -0300| [bb033e69107b5baad5fcec63bd8cd0b6908a2041] | committer: James Almer avcodec/ac3dec: only export matrix encoding and downmix info side data when necessary Don't export a matrix encoding side data when there's none signaled.

[FFmpeg-cvslog] avcodec/ac3dec: don't override existing downmix coefficient with default ones

ffmpeg | branch: master | James Almer | Wed Jan 15 22:12:28 2025 -0300| [4563cf95caeaa1f1bcb263b1ef6bb16932357903] | committer: James Almer avcodec/ac3dec: don't override existing downmix coefficient with default ones With this, if the eac3 dependent frame doesn't have coded downmix values when