C23 made these generic functions that no longer cast const
away for you, leading to warnings when compiling with C23
and a recent enough toolchain (glibc supports this since 2.43).
This commit fixes all the warnings that can simply be fixed
by adding const, without adding casts.
(the latter excludes parse_forced_key_frames() in ffmpeg_mux_init.c).
Reviewed-by: Kacper Michajłow <kasper93@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
97cbffe917