lavfi/buffersrc: set channel layout if it is known.

lavfi/buffersrc: set channel layout if it is known.

If buffersrc was configured for frames with an unknown layout,
the incoming frames will have channel_layout = 0.
If the format negotiation has selected a known (and compatible)
channel layout for the link, the frame is assumed to have
that layout, the field must be set before injecting the frame
in the filters.