Wired to Filter::Flush().

Closes #153

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The old formula calculates the output buffer size with "regular"
rounding (to the nearest integer), however sometimes, that is
insufficient and the last sample cannot be resampled.  This causes
audible distortions.  By changing the formula to consider the worst
case (always round up), this problem is eliminated.