Drop the swr_set_channel_mapping() + transpose_order() + reorder_in[]
plumbing. Instead, build the swr input channel layout directly from the
mpv chmap as AV_CHANNEL_ORDER_CUSTOM, so the labels in the layout already
match the positions of the channels in the input data.
For all unknown map, we return all UNSPEC, so the swr expands to default
layout for this channel number.
Kacper Michajłow
f5d4d9b029