The previous implementation could emit 'wildcard' channel layouts in too
many scenarios.
A wildcard channel count is -1 or -2, and indicates that any number of
channels is supported on the bus.
If the input and output layouts must match, then a layout of [-1, -1]
should be returned.
If any layout is valid in both directions, then a layout of [-1, -2]
should be returned.
In the case where we have a bus A and opposite bus B, we will now only
emit a wildcard count for bus A if every bus standard bus layout up to a
channel count of 16 can be applied successfully without changing the bus
count of B.