GetMediaInfo previously only recognized mpeg2video/h264/hevc and silently
returned 0x0 with no error for other codecs (VC-1, MPEG-4 ASP, AV1,
ProRes), causing DetectMediaType to misclassify as DVD and feeding bogus
dimensions to the zscale filter. Pick the first stream with
codec_type=video and return an explicit error if none is found.
exec.Command does not invoke a shell, so the wrapping " characters
were inserted into the muxed tag value verbatim (e.g. TITLE read as
"Snatch" instead of Snatch). Use unquoted Sprintf format strings.
The previous detector substring-matched "TFF"/"BFF" against idet's own
label text, so it returned true on every source, and `cmd.Start()` was
never paired with `Wait()`, leaving a zombie ffmpeg per file.
Run idet bounded with `-frames:v 400 -an -sn -f null -` so it completes,
use CombinedOutput so the child is reaped, and parse the "Multi frame
detection" summary line — interlaced only when TFF+BFF > Progressive.
Esa Kataja