dec->fmt_out can be written from any thread by the decoder module. It's only
safe to access it without any lock from the pf_vout_format_update() callback.

In order to fetch the last video_format_t from the decoder, we use the one from
decoded pictures (that are created from dec->fmt_out.video from the same thread
than pf_vout_format_update()).

Note for conversion_video_filter_append():
We feed to the video converter chain a new fmt created from the pic
video_format_t instead of the original dec->fmt_out. Normally, video filters
only use fmt*.video