Doing this with REDUCE_FORMATS() instead of swap_color_*() is not only
shorter, but more importantly comes with the benefit of being done
inside a loop, allowing us to correctly propagate complex graphs
involving multiple conversion filters (e.g. -vf scale,zscale).

The latter family of swapping functions is only used to settle the
best *remaining* entry if no exact match was found, and as such was
never the correct solution to YUV colorspaces, which only care about
exact matches.