page accumulator actually contains packets from the new GOP.
Previously we always included the last page with granulepos
from the previous GOP, in case it also contained the start of a
new packet. We now explicitly check if we can rule out this
possibility by remembering whether the successive page was
continued: we only include the earlier page if so.
This avoids adding preceding garbage where possible, ie. if
the keyframe starts a new page. This is uncommon with current
Theora encoders, but it is the normal (advised) case for Dirac.

git-svn-id: http://svn.annodex.net/liboggz/trunk@3880 8158c8cd-e7e1-0310-9fa4-c5954c97daef