Ability to filter side data when remuxing and retranscoding

[Mattias Wadman]

Hello, i've run into a use case were it would be useful to be able to filter out all or specific side data information during remux or retranscode. The specific side info that I would like to not copy in this case is an AV_PKT_DATA_SPHERICAL
display matrix that causes problems in another system.

Unfortunately i can't share the example file publicly but I will try find another one or see if it's ok to share it.

Relevant code references:

Side info copy for remux:
​https://github.com/FFmpeg/FFmpeg/blob/master/fftools/ffmpeg.c#L3158

Side info copy for retranscode:
​https://github.com/FFmpeg/FFmpeg/blob/master/fftools/ffmpeg.c#L3625

[Mattias Wadman]

Maybe worth to mention that in this case the display matrix is more or less an identity matrix that should not be needed for playback.