A call to swr_set_compensation() configures the resampler to drop or
duplicate "diff" samples over an interval of "distance" samples.
If the function is not called again, then after "distance" samples, no
more compensation will be applied. So it must always be called, even if
the new computed diff value happens to be the same as the previous one.
In practice, it is unlikely that the diff value is exactly the same
every second, except when it is actively clamped (to 2% of the sample
rate).
e9dd0f68ad