[cairo] Hairline support in Cairo

[Rick Yorgason]

Hey everyone,

I just pushed a PR to Inkscape that work in concert with my Cairo PR to 
add hairline support to both of them, and I was hoping to get a little 
visibility.

I've got a video showing what I'm doing here: 
https://www.youtube.com/watch?v=bNl8OiYRKVo&feature=youtu.be

My Cairo PR is here: 
https://gitlab.freedesktop.org/cairo/cairo/merge_requests/21

My Inkscape PR is here: 
https://gitlab.com/inkscape/inkscape/merge_requests/846

What my cairo patch does is add the cairo_set_hairline function to 
specify when things should be drawn as a hairline. On pixel surfaces, 
this usually means 1px lines, no matter what your zoom level (ie, 
transform) is. PS/PDF and some printers (or exotic devices, like laser 
cutters and plotters) have built-in support for representing hairlines.

Cheers!

-Rick-