vorbisdec: Initialize decoder directly once we have the 3 headers

vorbisdec: Initialize decoder directly once we have the 3 headers

... instead of waiting for the first non-header buffer.

Also drop non-identification headers arriving after initialization or
before the identification header. We don't do anything with them and
they would just accumulate.

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