Tweak new animation driver.

We're removing the bad/reallyBad concept for the benefit of an
accumulated lag. When the lag passes over a certain threshold, we
switch to time based. The logic for switching back remains unchanged.

We also fixed the switching so that elapsed() does not jump from the
predicted time to the animation system's wall time, but rather
continues from the predicted time with a walltime offset (this is how
it was always intended to be).

Task-number: QTBUG-42020
Change-Id: I7ee9181aca46cbc18a74fae5e8d513365906c017
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>