subviewer: sanitize packets.

The data does not contain timing or trailing line breaks anymore. In
addition to being less idiotic, it is consistent with other codecs and
thus allows more switches between formats and codecs. It also fixes the
issue of the trailing line returns being simple \n instead of CRLF in
the ASS rectangle dialogue (this is the reason of the FATE update).
lavf/webvtt: use ff_subtitles_read_chunk().

The FATE test is updated because the line breaks in the dialogue
rectangles are now proper CRLF.