[Tremor] what is this parameter (int decodep) use for ?

[xiphmont at xiph.org]

vorbis_synthesis can be called to simply keep the internal bitstream
tracking up to date without actually decoding the audio in the packet.
 This would be used to drop a few frames (for whatever reason) or
potentially fast-forward through a small segment of decoding without
having to spend time on decoding the audio.

In those cases, decodep would be 0.

Monty