POC extract parse index

[msg7086]

No description provided.

[Asd-g]

From quick test with double parsing ~1.61s, without double parsing ~1.51s.

You can set default value for the audio gaps and may be revert back the LWINDEX_INDEX_FILE_VERSION to 17.

This could be used to avoid the double parsing (https://github.com/HomeOfAviSynthPlusEvolution/L-SMASH-Works/blob/feature/extract_parse_index/common/lwindex.c#L3411):

const int video_indexing = opt->force_audio_index == -2;
lwindex_data_t *data = lwindex_parse(index, video_indexing, !video_indexing);