bug#8863: 23.3; Strikethrough won't display on MacOS

[Alp Aker]

> Dave Abrahams <dave@boostpro.com> writes:
>
> No, but the enclosed 2 patches, together, seem to.  Would somebody apply
> these, please?

I mentioned some problems with Chong Yidong's suggestion, one of which
still applies here: this patch won't work with stretch or image glyphs
(so, e.g., it won't do strike-through on tab characters).  

Also (something I didn't mention in the earlier post), this uses the
same Quartz drawing API as the existing code for underlining, and so
suffers from a problem that exists in that case as well; this kind of
line-drawing is subject to anti-aliasing effects that will cause lines
whose requested thickness is 1 pixel to appear thicker than that
(specifically, the line-drawing commands in question stroke lines so
that they're centered on their specified paths, and so a line whose
thickness is an odd number of pixels is dithered, making it look
thicker).

Last week I promised a patch that would implement strike-through and
overline face attributes and address problems with the existing
implementation of underlining.  I'll try to finish that up and post it
tomorrow.