[embedthumbnail] add new cli option '--force-convert-thumbnails'

yt_dlp/options.py

@@ -1748,6 +1748,11 @@ def create_parser():
             f'(currently supported: {", ".join(sorted(FFmpegThumbnailsConvertorPP.SUPPORTED_EXTS))}). '
             'You can specify multiple rules using similar syntax as "--remux-video". '
             'Use "--convert-thumbnails none" to disable conversion (default)'))
+    postproc.add_option(
+        '--force-convert-thumbnails',
+        dest='force_convert_thumbnails', action='store_true', default=False,
+        help=(
+            'Convert the thumbnails to another format even if it\'s already in this format'))
     postproc.add_option(
         '--split-chapters', '--split-tracks',
         dest='split_chapters', action='store_true', default=False,