Support simplified z-axis

[christianparpart]

An arbitrary Z-axis like Kitty would probably be overkill, especially because it puts a high burden on terminal authors that do not have a 3D API like OpenGL for rendering.

But reducing it to the minimal useful set should get the most out of it.

• render above text (basically like Sixel)
• replace text (EDIT: turns out this isn't easy for all TE devs? subject to removal)
• below text

For maximum compatibility with existing VT sequences, any screen buffer mutating VT sequence that affects text, will also affect image fragments. If a column is moved, its image fragment (regardless of z-axis) will be moved, too.

[christianparpart]

@dankamongmen, if I remember correctly (TMT), you where one amongst those raising interesting in having the ability to not just draw text above text, but also below. As I said, I generally dislike the idea of introducing an arbitrary z-axis due to its implied complexity on the TE side, I wanted to provide something similar, yet much simpler. Would you (in the context of notcurses and friends) be fine with above/below text for images, or would you want to need more than that?