[theora] Indexing Ogg files for faster seeking

[ogg.k.ogg.k at googlemail.com]

> I'm not sure about this. If your seek terminated at one of these
> "duplicate previous packet" packets, wouldn't you still need to load the
> original Kate packet in order to render it? If the original packet isn't

These are almost verbatim copies of the original packet, and are emitted
at user selected intervals, when the lifetime of an event is higher than that
interval. Thus, if you search at one of those, you don't have to seek to its
original packet, as you have all the data. The point of these is to avoid the
back seek (well, to bound the amount of data to seek back), so seeking to
a repeat packet is sufficient, even if the original event was much earlier.