vorbisdec: Always handle in-band header packets once the first non-header packet...

vorbisdec: Always handle in-band header packets once the first non-header packet arrives

And clean up any old pending headers if we receive a new identification
header, or if we receive a new set of headers via caps.

Otherwise it might happen that we receive one or more header but not
all, and then afterwards all headers again, and libvorbis does not like
getting headers passed multiple times and would error out.

It only makes sense to pass the very latest headers to the decoder at
the time we can actually make use of them.

https://bugzilla.gnome.org/show_bug.cgi?id=796980