Animation of layers during zoomToRect:animated:

if currently in a "float" zoom value, when using double tap of tap with two fingers, zoomToRect:animated: is called with a factor != 2. This will cause the zoomToRect:animated: animation duration to be different from 30ms.
That PR fixes correctPositionOfAllAnnotationsIncludingInvisibles to take that into account.
To test it simply zoom a little bit with a pinch gesture. Then double tap.
- before : layers doesn't animate correctly
- after: they do