Drawing UI elements behind text

[Cecilio Pardo]

Hello,

I am working on a mechanism to draw things on the background, behind
the text of a buffer.

This would allow to draw UI elements (such as indent lines or a fill
column indicator) and eye candy like background images or colored
windows.

This two first features (indent lines and fill indicator) are controlled
by declaring a list of vertical segments linked to buffer
positions.

Is there any other feature that could be implemented like this? Not just
by segments, but by any graphics drawn behind the text, vectors or
bitmap.

Thanks.