bug#20628: 25.0.50; Incorrect line height for some fonts

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 22 May 2015 23:49:59 +0200 (CEST)
> Cc: clement.pitclaudel@live.com, ohwoeowho@gmail.com, 20628@debbugs.gnu.org
> From: Werner LEMBERG <wl@gnu.org>
> 
> IMHO the bes solution is to completely ignore font-wide ascender and
> descender values.  Instead, use the TeX approach: set the line gap to
> the current size of the font, multiplied by a factor of 1.2 (and make
> this configurable on a font-by-font basis in case it isn't already),
> and increase the linegap if individual glyphs need it.

Could you perhaps look at the Emacs sources and suggest how to change
the *_open functions in the *font.c back-ends, to do what you suggest
above?  Or at least tell how to get "the current size of the font"
from the back-ends we use, which are Freetype, Fontconfig, and XLib's
XLoadQueryFont?  The relevant source files are xfont.c, ftfont.c, and
xftfont.c.

Also, how to know from the glyph metrics, in their Emacs incarnation,
that an individual glyph needs an increase of the vertical space?

Thanks.