Support input-slave srt in m3u playlists

[nyakze]

Currently MPC-HC doesn't load external subtitles in the input-slave when files are not hosted locally. Example playlist:
#EXTM3U #EXTINF:0,Star.trek.TOS.s02e01.Amok.Time.mkv #EXTVLCOPT:input-slave=http://localhost:8090/stream/Star.trek.TOS.s02e01.Amok.Time.srt# http://localhost:8090/stream/Star.trek.TOS.s02e01.Amok.Time.mkv

What should happen:
The player should automatically load the external subtitles.

What does happen:
The player doesn't see the external subtitles.

Workaround:
I can load them manually using "Load Subtitles".

[adipose]

#EXTM3U

#EXT-X-INDEPENDENT-SEGMENTS

#EXT-X-MEDIA:TYPE=AUDIO,URI="64b896fa2c2bc76a3c82150a_0_en_128k.m3u8",GROUP-ID="audio_128k",LANGUAGE="en",NAME="English",AUTOSELECT=YES,CHARACTERISTICS="DEFAULT=YES,AUTOSELECT=YES",CHANNELS="6"
#EXT-X-MEDIA:TYPE=AUDIO,URI="64b896fa2c2bc76a3c82150a_1_tr_128k.m3u8",GROUP-ID="audio_128k",LANGUAGE="tr",NAME="Turkish",AUTOSELECT=YES,CHARACTERISTICS="DEFAULT=NO,AUTOSELECT=YES",CHANNELS="6"
#EXT-X-MEDIA:TYPE=AUDIO,URI="64b896fa2c2bc76a3c82150a_1_tr_96k.m3u8",GROUP-ID="audio_96k",LANGUAGE="tr",NAME="Turkish",AUTOSELECT=YES,CHARACTERISTICS="DEFAULT=NO,AUTOSELECT=YES",CHANNELS="6"
#EXT-X-MEDIA:TYPE=AUDIO,URI="64b896fa2c2bc76a3c82150a_0_en_96k.m3u8",GROUP-ID="audio_96k",LANGUAGE="en",NAME="English",AUTOSELECT=YES,CHARACTERISTICS="DEFAULT=YES,AUTOSELECT=YES",CHANNELS="6"

#EXT-X-MEDIA:TYPE=SUBTITLES,URI="64b896fa2c2bc76a3c82150a_0_en.m3u8",GROUP-ID="subs",LANGUAGE="en",NAME="English",AUTOSELECT=YES,CHARACTERISTICS="DEFAULT=YES,AUTOSELECT=YES"
#EXT-X-MEDIA:TYPE=SUBTITLES,URI="64b896fa2c2bc76a3c82150a_1_tr.m3u8",GROUP-ID="subs",LANGUAGE="tr",NAME="Turkish",AUTOSELECT=YES,CHARACTERISTICS="DEFAULT=NO,AUTOSELECT=YES"

#EXT-X-STREAM-INF:BANDWIDTH=3294249,AVERAGE-BANDWIDTH=3111435,CODECS="avc1.4d401f,mp4a.40.2",RESOLUTION=1280x720,FRAME-RATE=23.976,VIDEO-RANGE=SDR,AUDIO="audio_128k",SUBTITLES="subs",CLOSED-CAPTIONS=NONE
64b896fa2c2bc76a3c82150a_0_720p.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=1314891,AVERAGE-BANDWIDTH=1225876,CODECS="avc1.4d401e,mp4a.40.2",RESOLUTION=854x480,FRAME-RATE=23.976,VIDEO-RANGE=SDR,AUDIO="audio_128k",SUBTITLES="subs",CLOSED-CAPTIONS=NONE
64b896fa2c2bc76a3c82150a_0_480p.m3u8

#EXT-X-STREAM-INF:BANDWIDTH=3265349,AVERAGE-BANDWIDTH=3079373,CODECS="avc1.4d401f,mp4a.40.2",RESOLUTION=1280x720,FRAME-RATE=23.976,VIDEO-RANGE=SDR,AUDIO="audio_96k",SUBTITLES="subs",CLOSED-CAPTIONS=NONE
64b896fa2c2bc76a3c82150a_0_720p.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=1285991,AVERAGE-BANDWIDTH=1193814,CODECS="avc1.4d401e,mp4a.40.2",RESOLUTION=854x480,FRAME-RATE=23.976,VIDEO-RANGE=SDR,AUDIO="audio_96k",SUBTITLES="subs",CLOSED-CAPTIONS=NONE
64b896fa2c2bc76a3c82150a_0_480p.m3u8

#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=203331,AVERAGE-BANDWIDTH=71440,CODECS="avc1.4d401e",RESOLUTION=854x480,VIDEO-RANGE=SDR,CLOSED-CAPTIONS=NONE,URI="64b896fa2c2bc76a3c82150a_0_480p_iframe.m3u8"
#EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=546112,AVERAGE-BANDWIDTH=146334,CODECS="avc1.4d401f",RESOLUTION=1280x720,VIDEO-RANGE=SDR,CLOSED-CAPTIONS=NONE,URI="64b896fa2c2bc76a3c82150a_0_720p_iframe.m3u8"

This is a similar problem but it is not using input-slave.

#2707

[clsid2]

That looks like an URL that will be send directly to LAV Splitter.

[adipose]

input-slave is apparently a VLC option.

Similar request for sub-file here for local files rejected for MPV: mpv-player/mpv#3877

input-slave is not very common, and if it's VLC specific, I don't see why we would want to support it.

[adipose]

I see that EXT VLC OPT stands for VLC Options as well.