lazy-loading per slide

[blueyed]

The current lazy-loading mechanism loads all elements for a section, but it would be better to do this per slide.

In #1289 (comment) you mention a planned lazyDistance feature, which is probably meant to pre-load elements?

In my case I have 30 images (in 30 slides), and it would be great if only the current and prev/next images would be loaded.

[blueyed]

#1338 got merged.
I've added #1340 as a follow-up with a new lazyDistance option (to be discussed).