Fix for slide transitions when changing direction

[mortenbekditlevsen]

The transition for removal used by SwiftUI
is the one that was set when rendering the slide.

This means that when changing navigation direction,
the animation would be wrong for the first transition
after changing direction.

By first updating the transition (causing a re-render
that doesn't change anything but the transition)
And then - in the next render loop - changing the
slide index, then we get the appropriate transition
even when changing directions.

This could be optimized to only perform the sleep
upon changing directions - by remembering the previous
transition direction and testing to see if it's
necessary to change the transition and re-render.