The previous scale expression `min(iw,iw*H/ih)':'min(ih,H)` produced odd
widths (e.g. 1425×720 for a 16:9 source capped at 720p) which libx264
refuses with `width not divisible by 2`, killing the encoder before a
single byte was written.
Switch to `scale=-2:H:force_original_aspect_ratio=decrease`, which
derives a width that preserves aspect ratio AND is rounded to a multiple
of 2. Always set `-pix_fmt yuv420p` so 10-bit HEVC sources are downcast
to the 8-bit format browser <video> elements actually decode.
Also add `-y`, guard nil pipe in Close(), and the related transcode
plumbing for browser-decided per-session quality.
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