bug#51821: 29.0.50; Suggest add variable or frame parameter: line-height

[tumashu]

>> maybe the height should split two parts, ascent and decent: 
>> 
>>    real-ascent = ascent * line-height-factor
>>    real-decent = decent * line-height-factor
>> 
>> The patch seem to not change decent.
>
>Why do you want to change the descent?
>

when we set fonts and let  1 Chinese char width = 2 * ASCII char width,
ASCII font size may different from Chinese font size, so maybe they are have 
different baseline.

so how to align to baseline, I do not know.