inside_ogg: store the absgp of the first page and tell people where the stream started

[est31]

There is a get_last_absgp function on the OggStreamReader. It returns the absgp of the last read page. That number can't be used for absolute measurements though, as the first audio page in an ogg/vorbis stream can start with any given absgp. To counter this, we need to remember the absgp of the first audio page in the stream, and use it as reference point. For increased accuracy, it would be best to take the actual begginning absgp, which is the absgp of the first audio page minus the sum of all sample counts of the ending packets in that page.