This fixes dynamic hwdec switching, which would often fails, because
hw decoder were feed data without context, which caused decoding errors,
which in turn disabled hwdec.
command.c does queue_seek() after decoder reinit, but it may already be
too late, by the time seek to keyframe finishes.
Fix this, by just discarding the data until we have safe point to start
decoding again. Limit this only for hwdec to avoid formats that may not
have ffmpeg parser and not give us keyframe information and limit to 96
frames, just as safety to not lock the player, but this shouldn't happen
in practice.
This should also fix cases, when media file is cut in a way that would
cause decoding errors at the start.
Fixes: #17484
Kacper Michajłow
7ec8a7e9b1