Suppoprt "stacked" mode for MultiMedium

[llucax]

A nice way to build content using a MultiMedium can be to instead doing a slide-show, where the previous medium in the track is replaced by the new one, it is stacked on top of it.

• For images this means stacking one image on top of the previous one(s).
• For videos it might be hard to make sense of it, as I'm not sure there are transparent videos (maybe GIFs).
• For audios it means not stopping the previous audio.