Current method for constraining fragment position drift suffers from
round-off error build up.

Instead of calculating cumulative drift as a sum of input fragment
position corrections, it is more accurate to calculate drift as the
difference between current fragment position and the ideal position
specified by the tempo scale factor.
Signed-off-by: Pavel Koshevoy <pkoshevoy@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>