avcodec, avutil, avformat: remove AVOption requirement for some fields

avcodec, avutil, avformat: remove AVOption requirement for some fields

Allow all struct fields to be accessed directly, as long as they're
public.

Before this change, many fields were "public", but could be accessed via
AVOption only. This meant they were effectively not public, but were
present for documentation purposes, which was incredibly confusing at
best.