according to the spec, a vorbis page may be 255*255+header bytes
big.
_get_prev_page() only seeks backwards by CHUNKSIZE bytes, where
CHUNKSIZE is defined to be 8500 bytes.
that means, _get_prev_page() may fail to find the previous
page eventhough it's operation on a spec conforming ogg stream.
though it's prolly inefficient for _get_prev_page() to seek
back by ~64k by default, it should be able to revert to that
much backstepping if nothing is found in the first 8500, so as
to work correctly on conforming ogg streams.
