Filters mostly work in native endianness, but they must output
a specified endianness, usually little: that requires a final
conversion for big endian.
I do not know what's the deal with gif-deal: inserting explicitly
the filters that are implicitly inserted result in less frames in
output. Probably a strange problem of duration.
Explicitly insert the scale or aresample filter where it would
have been inserted by the negotiation.
Re-enable conversions if it cannot be done easily.
If a conversion is needed in a test, we want to know about it.
If the negotiation changes and makes new conversion necessary,
we want to know about it even more.
