The code will currently add a small offset to avoid exact midpoints, but
this can cause inexact results when a float timestamp is exactly
representable as an integer.
Fixes off-by-one in the first frame duration in multiple FATE tests.
Previously they would only be used with trivial filtergraphs, because
filters did not handle frame durations. That is no longer true - most
filters process frame durations properly (there may still be some that
don't - this change will help finding and fixing them).
Improves output video frame durations in a number of FATE tests.
