The old logic was trying to be excessively clever in "deducing" that the
user wanted to stretch/scale the image when ow/oh differed from iw/ih
aspect ratio. But this is almost surely unintended except in
pathological cases, and in those cases users should simply disable
normalize_sar and do all the stretching/scaling logic themselves. This
is especially important in multi-input mode, where the canvas may be
vastly different from the input dimensions of any stream. Also, passing
through input 0 SAR in multi-input mode is arbitrary and nearly useless,
so again force output SAR to 1:1 here.