Render a buffer or string to a simpler string?

[Dmitry Gutov]

I'm currently fixing Company's popup overlay positioning and display
interacts with org-indent-mode. The latter adds `line-prefix' text
properties everywhere in the buffer, and they influence the offset of
each line of the popup "rectangle".

Is there an easy way to convert a string with `line-prefix' and,
optionally, `prefix' and `display' properties to a string without those
properties corresponding to how the former string is going to be
displayed?

Or, instead of a string, convert a buffer?

Somewhat relatedly, is there a way to make `count-screen-lines' work in
batch mode (i.e. not return crazy numbers)? It would be nice to be able
to run code that calls it on an integration server.