bug#30609: 26.0.91; underline should be drawn behind text

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Sun, 25 Feb 2018 15:05:29 -0800
> 
> I don't know if it's true on all platforms, but at least on the mac the
> text decorations (like underlines) are drawn after the glyph causing
> them to overlap the descenders. Typically underlines are drawn behind
> the characters as they are less important than the characters
> themselves.
> I'd be happy to submit a patch to change the order if no one sees a problem 
> with this.

I'm far from being an expert in this area, but AFAICT, such a change
could cause the underline to become partially or even fully invisible,
at least in some situations.  For example, if the underline is to be
drawn below one or more images, or some special characters, or what we
use to display characters with no font.  That's because we sometimes
clear the entire character cell with the background color before
drawing the foreground, and that would erase the part of the underline
below that cell.  Right?