CodeRabbit's leftover nitpick: h264_mp4 hard-coded -preset medium -crf 18 for
whatever encoder the build ships, but libopenh264 (and the hardware backends)
reject both — so a build whose picked encoder is not libx264 failed the fixture
and silently skipped every check gated on it. libx264 keeps its constant-quality
pair; everything else gets -b:v. Pinned by a small test over the option sets.
Kaedo17
949736fe36