Add compatibility with FFMPEG 7.0

[eclipseo]

avio: Constify data pointees of write callbacks

Ref: https://github.com/FFmpeg/FFmpeg/commit/2a68d945cd74265bb71c3d38b7a2e7f7d7e87be5 https://github.com/FFmpeg/FFmpeg/commit/02aea61d69d8f81bc285e2131bf25f96a3e27feb