ytdl_hook: add script opt for using manifest URLs

Disable by default.
This feature was added in 7eb342757, which allowed stream selection
in runtime. Problem with this atm is that FFmpeg will try to demux
every first packet of every track leading to noticeable delay opening
the URL.

This option can be changed to enabled by default or removed when
HLS/DASH demuxers are improved upstream.

DOCS/man/options.rst

@@ -560,6 +560,10 @@ Program Behavior
     which mpv should not use with youtube-dl. The patterns are matched after
     the ``http(s)://`` part of the URL.
 
+    The `use_manifests` script option makes mpv use the master manifest URL for
+    formats like HLS and DASH, if available, allowing for video/audio selection
+    in runtime. It's disabled ("no") by default for performance reasons.
+
     ``^`` matches the beginning of the URL, ``$`` matches its end, and you
     should use ``%`` before any of the characters ``^$()%|,.[]*+-?`` to match
     that character.

player/lua/ytdl_hook.lua

@@ -4,7 +4,8 @@ local options = require 'mp.options'
 
 local o = {
     exclude = "",
-    try_ytdl_first = false
+    try_ytdl_first = false,
+    use_manifests = false
 }
 options.read_options(o)
 
@@ -279,7 +280,7 @@ local function add_single_video(json)
     local reqfmts = json["requested_formats"]
 
     -- prefer manifest_url if present
-    if valid_manifest(json) then
+    if o.use_manifests and valid_manifest(json) then
         local mpd_url = reqfmts and reqfmts[1]["manifest_url"] or
             json["manifest_url"]
         if not mpd_url then