youtube.lua: &fmt={audio,video} URL parameters for "adaptive" formats

This extends the old &fmt=[itag] URL syntax that we still supported all
along to force format selection, and allows choosing from "adaptive"
elementary streams without knowledge of itag specifics and in accordance
with normal resolution preferences.

This also allows playing only the audio part of music videos and
skipping the download of the video part entirely, greatly reducing
bitrate and providing mitigation against the throttling issue.

Ref #10237, #27227

share/lua/playlist/youtube.lua

@@ -655,9 +655,9 @@ function pick_stream( formats, fmt )
     -- Remove subobject fields to ease parsing of stream object array
     formats = string.gsub( formats, '"[^"]-":{[^{}]-},?', '' )
 
-    fmt = tonumber( fmt )
-    if fmt then
+    if tonumber( fmt ) then
         -- Legacy match from URL parameter
+        fmt = tonumber( fmt )
         for stream in string.gmatch( formats, '{(.-)}' ) do
             local itag = tonumber( string.match( stream, '"itag":(%d+)' ) )
             if fmt == itag then
@@ -673,8 +673,14 @@ function pick_stream( formats, fmt )
         for stream in string.gmatch( formats, '{(.-)}' ) do
             local height = tonumber( string.match( stream, '"height":(%d+)' ) )
 
+            -- We have no preference mechanism for audio formats,
+            -- so just pick the first one
+            if fmt == "audio" and not height then
+                return stream
+            end
+
             -- Better than nothing
-            if not pick or ( height and ( not bestres
+            if ( not pick and fmt ~= "video" ) or ( height and ( not bestres
                 -- Better quality within limits
                 or ( ( prefres < 0 or height <= prefres ) and height > bestres )
                 -- Lower quality more suited to limits
@@ -696,10 +702,21 @@ function pick_stream_url( muxed, adaptive, js_url, fmt )
         vlc.msg.warn( "Couldn't extract YouTube JavaScript player code URL, descrambling functions unavailable" )
     end
 
-    local pick = pick_stream( muxed, fmt )
-    if not pick and tonumber( fmt ) then
+    local pick = nil
+    if tonumber( fmt ) then
+        -- Specific numeric itag, search in both lists
+        pick = pick_stream( muxed, fmt )
+        if not pick then
+            pick = pick_stream( adaptive, fmt )
+        end
+    elseif ( fmt == "audio" or fmt == "video" ) then
+        -- Specifically audio or video only, no fallback
         pick = pick_stream( adaptive, fmt )
+    else
+        -- Default: multiplexed streams
+        pick = pick_stream( muxed, fmt )
     end
+
     if not pick then
         return nil
     end
@@ -784,7 +801,7 @@ function parse()
         local path, title, description, artist, arturl, js_url
 
         -- Retired YouTube API for video format itag parameter,
-        -- still supported as youtube.lua API
+        -- still supported and extended as youtube.lua API
         -- https://en.wikipedia.org/w/index.php?title=YouTube&oldid=716878321#Quality_and_formats
         local fmt = get_url_param( vlc.path, "fmt" )