[theora] Indexless interpolation search seeking code

[xiphmont at xiph.org]

> sure, but  you're looking at around 5-7 seeks for a random spot,
> whereas you pretty much have 1 by definition with an index ...

The first seek in an index gets you within a few seconds, then you
have to search from there anyway.  An interpolated bisection search
using that same granularity would vary depending on how varied the
bitrate changes in the file would be.  5-7 would be on the high side,
but the number would be variable.  The index would give a more
predictable starting performance.

Again, this all assumes that we're seeking to find some specific frame
number, audio sample position, etc.  That's considerably more work for
both seeking cases than finding an approximate point, or moving
forward/backward an approximate amount.

Monty