[cairo] Patch to allow Cairo-based software to print to laser cutters on Windows

[Carl Worth]

On Wed, Oct 17 2018, Rick Yorgason wrote:
> With this patch, I can use Inkscape to set my stroke width to 0.001" and 
> it will cut through the material as expected.

If we're going to have a magic value here, I think I'd be much happier
to have 0 be the magic value.

(I know that your code isn't actually making 0.001" be magic, but a
whole range of values. I think I would really prefer to have that range
include rather then exclude 0.0.)

Does anyone see any reason why a stroke width of 0 shouldn't be treated
this way?

I think that would make a much better way to be able to document
this. ("Use a value of 0 to get a PS_COSMETIC pen which is useful when
targeting devices such as laser cutters".)

And speaking of documentation, it seems this patch should also be
touching up the documentation, such as in cairo_set_line_width?

I know the code is specific to the Windows backend, but I think it's
reasonable to put a backend-specific note into the general documentation
in a case like this.

-Carl
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