Bug #2: calculateZscaleWidth previously emitted a zscale filter even
when no rescale was needed (SAR 1:1, N/A, empty, or computed width
equal to the source). encodeVideo then unconditionally appended it to
-vf, forcing a pointless colorspace round-trip. Return an empty filter
string in those cases and build the -vf chain conditionally; omit -vf
entirely when no filters apply.
Bug #23: the SAR-to-width math used integer truncation, producing odd
or off-by-one widths (e.g. 853 instead of 854 for 32:27 at 1920), and
the guard accepted SAR 0:N which zeroed the output width. Reject
zero-numerator SARs and round to the nearest integer then mask to an
even width for AV1/H.264 mod-2 alignment.
Esa Kataja
71f7ef7bca