First attempt on z-caching.
Smoother scrolling through slices as a combination of caching and limiting the repeat rate of the keypress event.
Coordinate conversions work again when using linear zooming.
Switching between linear zooming and image zooming kind of works.
Combination of linear zooming in and zooming out with image tile loading now works.
Zooming out now works properly.
Increase the speed of linear zooming.
Merge branch 'linear_zoom'