Symphonia should support unpacking Vorbis extradata packed in the Xiph lacing format

[baylesj]

WebM and Matroska containers use the Xiph lacing format for Vorbis extradata, where a single CodecPrivate buffer contains all three Vorbis headers: the Identification, Comment, and Setup headers.

Symphonia's symphonia-codec-vorbis decoder does not seem to understand the Xiph packaging layer. It expects only the raw Identification and Setup headers laid out sequentially. This function unpacks the Xiph format and returns a new vector containing only those two required headers.