(quality takes higher priority)
-1 for default (order by other properties),
-2 or smaller for less than default.
- * http_method HTTP method to use for the download.
* http_headers A dictionary of additional HTTP headers
to add to the request.
- * http_post_data Additional data to send with a POST
- request.
* stretched_ratio If given and not 1, indicates that the
video's pixels are not square.
width : height ratio as float.