More precise chapter management with EDL

Using edl, it seems to be there is no way to keep the exact chapters of a source file without mpv manually adding one for the start of a entry.
This unnecessarily increases the chapter count and index by one in the statistics screen for externally provided chapters.
On v0.34.1 it also seems to overrides the title of my first provided chapter, but not on git-master.

I propose another edl header 'keep_source_chapters', to be used in conjunction with 'no_chapters'.

Current behavior

# mpv EDL v0
!no_chapters
some_file_01.m2ts
some_file_02.m2ts
some_file_03.m2ts
!new_stream
my_custom_chapter_file.ffmetadata

Given this file, mpv adds a chapter mark at the beginning (and changes its title to the file title).

Expected behavior of the wanted feature

# mpv EDL v0
!no_chapters
some_file_01.m2ts
some_file_02.m2ts
some_file_03.m2ts
!new_stream
!no_chapters
!keep_source_chapters
my_custom_chapter_file.ffmetadata

Given this file, mpv won't add a chapter mark at the beginning and only keep the chapters from the ffmetadata file.

If my explanation isn't clear enough i provide a small diff.
https://gist.github.com/sshiroi/dcab74ec7304ba37a268f97f7480d00d

Alternative behavior of the wanted feature

Some other way or option for mpv to only use my chapters.

No source file or test is named. Start by tracing EDL handling for the !no_chapters and !new_stream entries, then compare the current behavior with the provided ffmetadata example and the linked diff. Done means externally supplied chapters are preserved without an automatically added or renamed initial chapter.