ytdl_hook.lua: delay load subtitles

Uses the infrastructure added in the previous commits. This is
admittedly a bit weird (constructing EDL URLs and such). But on the
other hand, adding this as "first class" mechanism directly to the
sub-add command or so would increase weirdness and unexpected behavior
in other places, or at least that's what I think.

To reduce confusion, this goes through the effort of mapping the webvtt
codec, so it's shown "properly" in the codec list. Without this it would
show "null", but still work. In particular, any non-webvtt codecs should
still work if libavcodec supports it.

Not sure if I should remove the --all-subs hack from the code. But I
guess it does no harm.

player/lua/ytdl_hook.lua

@@ -29,6 +29,11 @@ local safe_protos = Set {
     "data"
 }
 
+-- Codec name as reported by youtube-dl mapped to mpv internal codec names.
+local map_codec_to_mpv = {
+    ["vtt"] = "webvtt",
+}
+
 local function exec(args)
     local ret = mp.command_native({name = "subprocess",
                                    args = args,
@@ -386,8 +391,13 @@ local function add_single_video(json)
             end
 
             if not (sub == nil) then
-                mp.commandv("sub-add", sub,
-                    "auto", sub_info.ext, lang)
+                local edl = "edl://!no_clip;!delay_open,media_type=sub"
+                local codec = map_codec_to_mpv[sub_info.ext]
+                if codec then
+                    edl = edl .. ",codec=" .. codec
+                end
+                edl = edl .. ";" .. edl_escape(sub)
+                mp.commandv("sub-add", edl, "auto", sub_info.ext, lang)
             else
                 msg.verbose("No subtitle data/url for ["..lang.."]")
             end