The code only set the channel layout of the AVFormatContext, so the user could
not override the channel layout if the demuxer did not have such parameter.

This used to work via the respective AVCodecContext option, but since
639c2f00 it no longer gets passed to the
decoders. It is actually better if we set it manually, instead of using the
codec option because that way we can also override it on the stream level, so
it will also work for stream copy or bitstream filtering.

We don't allow changing the number of channels, because that can cause
unexpected results. We disable layout guessing, if a channel layout is
specified.

Fixes ticket #11016.
Signed-off-by: Marton Balint <cus@passwd.hu>