[theora] Indexing Ogg files for faster seeking

[Carlos Jose Lenarts Ramis]

I think that better to an index per track is to have a global index, but if
it has each track index multiplexed, the main difference is that we don't
have n key-points wee have n key-points per track, then the number of
key-points are also related to the number of tracks and each key points is
marked, this add a little more of overhead. In this approach each track can
have a different number of key-points for example the key-points for a
subtitle marks the lines but in a video it marks each 500ms. The only
modification to the original idea is to ad a track identifier in the
key-points, the results is that key-point have this information:

the page offset as an 8 byte unsigned integer, followed by
the checksum of the page found at the offset, as a 4 byte field,followed by
the ther refered bitstream serial number of the track, as a 4 byte
field,followed by
the presentation times in milliseconds of the key point, as an 8 byte
unsigned integer.

To make a search in this approach is litle more complex as we have to check
that the refereed key-point if from a active track, but that complexity is
negligible. Also to make the index in this approach is more complex but not
much.
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