[formatsort] Remove misuse of 'preference'

'preference' is to be used only when the format is better that ALL qualities of a lower preference irrespective of ANY sorting order the user requests. See deezer.py for correct use of this

In the older sorting method, `preference`, `quality` and `language_preference` were functionally almost equivalent. So these disparities doesn't really matter there

Also, despite what the documentation says, the default for `preference` was actually 0 and not -1. I have tried to correct this and also account for it when converting `preference` to `quality`

youtube_dlc/extractor/imgur.py

@@ -72,7 +72,7 @@ class ImgurIE(InfoExtractor):
                 gif_json, video_id, transform_source=js_to_json)
             formats.append({
                 'format_id': 'gif',
-                'preference': -10,
+                'preference': -10,  # gifs are worse than videos
                 'width': width,
                 'height': height,
                 'ext': 'gif',