bug#18195: 24.3.92; window-screen-lines is not accurate

[Eli Zaretskii]

> Date: Tue, 05 Aug 2014 12:16:40 +0200
> From: martin rudalics <rudalics@gmx.at>
> 
> If things like line spacing are involved you really should calculate
> pixelwise.

He can't: we currently don't expose enough information for Lisp
programs to perform layout.  E.g., the metrics of glyphs and lines
that use arbitrary fonts and display images are not easily available
on the Lisp level, and neither are dimensions and line counts of
arbitrary regions of the screen (although you could write some clunky
Lisp to do the latter).  Likewise with dimensions of text in overlays
and display properties, and with images.

This was never supposed to be needed, and thus was never designed and
implemented.  If we want to allow such jobs from Lisp, we need to
close these gaps.