avfilter/formats: allow unknown channel layouts by default
Since the default in the libav fork is to only allow known layouts, making
unknown layouts allowed by default here can be a security risk for filters
directly merged from libav. However, usually it is simple to detect such cases,
use of av_get_channel_layout_nb_channels is a good indicator, so I suggest we
change this regardless.
See http://ffmpeg.org/pipermail/ffmpeg-devel/2016-November/203204.html.
This patch indirectly adds unknown channel layout support for filters where
query_formats is not specified: