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

[Eli Zaretskii]

> From: Oleh Krehel <ohwoeowho@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  clement.pitclaudel@live.com,  
> 20628@debbugs.gnu.org
> Date: Fri, 29 May 2015 16:49:41 +0200
> 
> Here's the rest of the info for "lambda" being composed into "lambda":
> 
>              position: 14095 of 224875 (6%), column: 4
>             character: l (displayed as l) (codepoint 108, #o154, #x6c)
>     preferred charset: ascii (ASCII (ISO646 IRV))
> code point in charset: 0x6C
>                script: latin
>                syntax: w      which means: word
>              category: .:Base, L:Left-to-right (strong), a:ASCII, l:Latin, 
> r:Roman
>              to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 RET NAME"
>           buffer code: #x6C
>             file code: #x6C (encoded by coding system utf-8-unix)
>               display: composed to form "lambda" (see below)
> 
> Composed with the following character(s) "ambda" by the rule:
>       (?λ)
> The component character(s) are displayed by these fonts (glyph codes):
>  λ: xft:-unknown-Latin Modern 
> Math-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1 (#xF6F)
> See the variable `reference-point-alist' for the meaning of the rule.

The key part is "Composed with" etc.  My changes didn't touch the
composed characters, so it's small wonder you see this.

I will try to have a look, once I get the other issues covered, but
don't hold your breath: I don't really understand all the complex
juggling we do with ascent and descent for composed characters, and
unless I find an easy and safe solution, I'd prefer to tell people not
to set these fonts as default fonts than to break character
composition, which is much more central to Emacs display capabilities.

Thanks.