* codecs: walk open-GOP leading pictures back in a loop, not by recursion
MxfVideo::sync_before and MpegVideo::sync_before recursed one GOP back
per leading picture, so a crafted index whose every GOP leads its key
frame recursed once per GOP: tens of thousands of frames for a small
file, a plausible stack overflow in dev builds.
Both now use gop::open_gop_sync, a loop over the same steps. It stops
when the index's sync sample lies past the picture instead of looping,
and reads pts and keys with get().
Fixes#2803
* gop: keep decode_start's doc comment on decode_start
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---------
Co-authored-by: Brandon Thomas <brandon@storyteller.ai>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
