Add an optional "wipe" transition between slides

[alykat]

could be useful for titlecards, quiz cards and key transitional moments

how i think the mongolia title cards are working but i need to dig into the code:

• the proceeding slide fades out / is replaced by a black box.
• in the background the title image has faded in, covered by the black box
• as you scroll, the black box scrolls up, revealing the title image
• the content of the title slide has a ton of padding on it so the title + map are not visible at first
• and then the chart underneath is either built into the title slide or has the same image as the bg
• and then there’s a fade at the bottom of it that for a little bit of fade/wipe transition to the next slide

suggested by @RKseen