ytdl_hook: use the new max_request_size AVOption if needed

Some sites throttle requests, but throttling can be avoided with
many small range requests. yt-dlp implements a "http_chunk_size"
key in its -J output to inform us of the ideal size.

With the appropriate[1] FFmpeg addition, we can properly implement
this now, which greatly speeds up how fast YouTube videos get loaded
into the cache by mpv.

Should the FFmpeg that mpv is built against lack the max_request_size
AVOption, then this will silently not do anything new, so no version
checks are needed.

Fixes #8655.

[1]: https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250101204126.748587-1-ffmpeg@fratti.ch/

player/lua/ytdl_hook.lua

@@ -894,6 +894,18 @@ local function add_single_video(json)
         stream_opts["cookies"] = serialize_cookies_for_avformat(existing_cookies)
     end
 
+    local chunk_size = math.huge
+    if has_requested_formats then
+        for _, f in pairs(requested_formats) do
+            if f.downloader_options and f.downloader_options.http_chunk_size then
+                chunk_size = math.min(chunk_size, tonumber(f.downloader_options.http_chunk_size))
+            end
+        end
+    end
+    if chunk_size < math.huge then
+        stream_opts = append_libav_opt(stream_opts, "max_request_size", tostring(chunk_size))
+    end
+
     mp.set_property_native("file-local-options/stream-lavf-o", stream_opts)
 end