This properly handles pre-skip, seeking pre-roll, and end-trim. It
uses the i_nb_samples field of block_t to signal to the decoder how
many samples should be returned after (possibly) skipping some at
the start of the block (for both pre-skip and pre-roll). In
addition, it abuses the i_length field of block_t to signal to the
decoder how many samples to trim from the end of the packet (for
end-trimming).

This patch does not compute correct timestamps for streams which
start at a non-zero offset (e.g., live streams joined in the
middle), nor does it correctly compute the length of a stream. But
neither of those things work for Vorbis, either. I'm leaving them
for a follow-up patch.

Signed-off-by: Rafaël Carré <funman@videolan.org>