youtube.lua: &fmt=hd URL parameter to leverage "adaptive" formats

This provides experimental support to play again all available video
resolutions, by combining audio and video "adaptive" elementary streams
using an input slave. Not in use by default; use at your own risks.

Again, by allowing to select resolutions lower than 360p, this also
provides mitigation against the throttling issue.

Ref #10237, #27227

share/lua/playlist/youtube.lua

@@ -713,8 +713,25 @@ function pick_stream_url( muxed, adaptive, js_url, fmt )
         -- Specifically audio or video only, no fallback
         pick = pick_stream( adaptive, fmt )
     else
-        -- Default: multiplexed streams
-        pick = pick_stream( muxed, fmt )
+        if fmt == "hd" then
+            -- Try and leverage full array of adaptive formats
+            local audio = pick_stream( adaptive, "audio" )
+            local video = pick_stream( adaptive, "video" )
+            if audio and video then
+                local audio_url = assemble_stream_url( audio, js )
+                local video_url = assemble_stream_url( video, js )
+                if audio_url and video_url then
+                    return video_url, audio_url
+                end
+            end
+        end
+
+        if not pick then
+            -- Default or fallback: safe old multiplexed streams,
+            -- but reduced to a single, low-definition format
+            -- available in some cases
+            pick = pick_stream( muxed, fmt )
+        end
     end
 
     if not pick then
@@ -804,7 +821,7 @@ function parse()
         or string.match( vlc.path, "/live%?" )
         or string.match( vlc.path, "/shorts/" )
     then -- This is the HTML page's URL
-        local path, title, description, artist, arturl, js_url
+        local path, path2, title, description, artist, arturl, js_url
 
         -- Retired YouTube API for video format itag parameter,
         -- still supported and extended as youtube.lua API
@@ -951,7 +968,7 @@ function parse()
 
                     if stream_map or adaptive_map then
                         vlc.msg.dbg( "Found new-style parameters for youtube video stream, parsing..." )
-                        path = pick_stream_url( stream_map, adaptive_map, js_url, fmt )
+                        path, path2 = pick_stream_url( stream_map, adaptive_map, js_url, fmt )
                     end
                 end
 
@@ -977,7 +994,11 @@ function parse()
             arturl = get_arturl()
         end
 
-        return { { path = path; name = title; description = description; artist = artist; arturl = arturl } }
+        local options = { }
+        if path2 then
+            table.insert( options, ":input-slave="..path2 )
+        end
+        return { { path = path; name = title; description = description; artist = artist; arturl = arturl; options = options } }
 
     elseif string.match( vlc.path, "/get_video_info%?" ) then
         -- video info API, retired since summer 2021