[fd/ffmpeg] Accept output args from info dict (#9278)

Authored by: bashonly

yt_dlp/downloader/external.py

@@ -491,7 +491,7 @@ class FFmpegFD(ExternalFD):
         if not self.params.get('verbose'):
             args += ['-hide_banner']
 
-        args += traverse_obj(info_dict, ('downloader_options', 'ffmpeg_args'), default=[])
+        args += traverse_obj(info_dict, ('downloader_options', 'ffmpeg_args', ...))
 
         # These exists only for compatibility. Extractors should use
         # info_dict['downloader_options']['ffmpeg_args'] instead
@@ -615,6 +615,8 @@ class FFmpegFD(ExternalFD):
         else:
             args += ['-f', EXT_TO_OUT_FORMATS.get(ext, ext)]
 
+        args += traverse_obj(info_dict, ('downloader_options', 'ffmpeg_args_out', ...))
+
         args += self._configuration_args(('_o1', '_o', ''))
 
         args = [encodeArgument(opt) for opt in args]

yt_dlp/extractor/common.py

@@ -246,7 +246,8 @@ class InfoExtractor:
                     * downloader_options  A dictionary of downloader options
                                  (For internal use only)
                                  * http_chunk_size Chunk size for HTTP downloads
-                                 * ffmpeg_args     Extra arguments for ffmpeg downloader
+                                 * ffmpeg_args     Extra arguments for ffmpeg downloader (input)
+                                 * ffmpeg_args_out Extra arguments for ffmpeg downloader (output)
                     * is_dash_periods  Whether the format is a result of merging
                                  multiple DASH periods.
                     RTMP formats can also have the additional fields: page_url,