FFmpeg v9 support: remove obsolete -vsync flag

[nim65s]

fix eg.:

      if not num_images:
        num_images = -1
      if not width:
>       raise RuntimeError(f'Unable to parse video header: {err}')
E       RuntimeError: Unable to parse video header: ffmpeg version 9.0.1 Copyright (c) 2000-2026 the FFmpeg developers
E         built with gcc 15.3.0 (GCC)
E         configuration: […]
E         libavutil      61.  1.101 / 61.  1.101
E         libavcodec     63.  1.101 / 63.  1.101
E         libavformat    63.  1.101 / 63.  1.101
E         libavdevice    63.  1.101 / 63.  1.101
E         libavfilter    12.  1.101 / 12.  1.101
E         libswscale     10.  1.101 / 10.  1.101
E         libswresample   7.  1.101 /  7.  1.101
E       Unrecognized option 'vsync'.
E       Error splitting the argument list: Option not found

This was deprecated in ffmpeg since 2023 and v7.0: FFmpeg/FFmpeg@3dc3195

And has been removed in v9.0: FFmpeg/FFmpeg@687e061