This is useful for parser like flacparse or h264parse which may need to process
some buffers before they can construct the final caps, in which case they may
want to delay pushing the initial buffers until the full and proper caps are
known.
https://bugzilla.gnome.org/show_bug.cgi?id=646341
This makes more sense conceptually, since the bitrate may be used
to estimate a seek position if there's no seek table or just for
duration reporting/estimation if we can't seek. Also, even if the
format is not syncable, we could still seek by pushing data from the
start and using the segment to make downstream clip.
https://bugzilla.gnome.org/show_bug.cgi?id=518857
Also change gst_base_parse_set_format(parse,flags,switch_on) to
gst_base_parse_set_format_flags(parse,flags) which is more in line
with the rest of our API and how the function is used.
