Allow loading .vtt from remote urls

[glensc]

This bypasses the need for a local server and the need to keep castnow command running.

---

This post here serves as a small blog post on how to play content on Chromecast with subtitles, without having need to keep castnow running. I used to re-encode media to .mp4 and burn subtitles into a video stream. this is not necessary.

I managed to get castnow to make Chromecast serve all content from remote servers:

1. a .mkv container with h264 video, Vorbis audio
2. .vtt subtitles on a remote server

Chromecast (ultra) plays fine .mkv container, just google chrome doesn't play it, so it's common to understand Chromecast can't support .mkv:

• https://www.reddit.com/r/ffmpeg/comments/6chy76/mkv_to_mp4_for_chromecast/dhwcbwt/

and the official docs don't mention Matroska (yet?):

• https://developers.google.com/cast/docs/media#media_container_formats

for audio, it's important it to be "channels: 2" aka, stereo, Chromecast won't play "5.1" audio. sometimes I've seen it not even opening media. sometimes it just plays video only.

for FFmpeg convert from 5.1, the extra options also include pan filter, otherwise, it would sound too loud or too low: -ac 2 -af "pan=stereo|FL=FC+0.30*FL+0.30*BL|FR=FC+0.30*FR+0.30*BR":

• https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg

as for .vtt serving, it's important:

1. have proper content-type: text/vtt; charset=utf-8
2. must-have Access-Control-Allow-Origin: * (or similar) header

for Lighttpd, this can be done with:

mimetype.assign += (
       ".vtt"         => "text/vtt; charset=utf-8",
)

# https://redmine.lighttpd.net/projects/1/wiki/Docs_ModSetEnv
server.modules += (
    "mod_setenv",
)
setenv.add-response-header = ( "Access-Control-Allow-Origin" => "*" )