Prevent QUnifiedTimer from ticking backwards.

This could happen in the following situation:
* a custom animation driver with fixed delta
* a triple-buffering scheme (rendering ahead a frame)
* a second animation timer starting while a first was active

This would cause QUnifiedTimer::startTimers() to trigger
QUnifiedTimer::updateAnimationTimers(-1), and use the current time from
the QElapsedTimer rather than the animation driver. This time could be
less than the last reported time from the animation driver.

Change-Id: Ibf1796fcb99f288d4946b30e5e7225695aa61781
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>