Support "incremental" display of elements that take up the whole screen

[nhooey]

Currently, you can have a slide with incremental bullets:

!SLIDE bullets incremental
# Title #
* Bullet 1
* Bullet 2

Where the bullets will display incrementally when you press "right" on the keyboard. But when you back up with "left", the entire slide in its final state will display, and then the previous entire slide will display. In other words, the bullets are incremental in one direction only.

Currently there is no way to display elements incrementally that take up the entire screen. So if you wanted to display a series of images that form an animation, there is no way to display them incrementally.

[jfinkels]

For that situation, you could just put each image on its own slide.

However, I would like to see incremental replacement of individual elements of a page, though I don't see how that's possible with Markdown syntax. The solution would be to just copy and paste the entire contents of each slide to a new slide, and then change the one element on the new slide which is supposed to change.