[cairo] thin antialiased lines

[Jeff Bezanson]

> have the same "problem" of too thick or blurry lines. One example is
> the statistical package R (www.r-project.org). Plotting a normal
> distribution looks very nice, while plotting a survival curve (which
> basically consists of more or less horizontal lines) looks blurry.

I would expect that for scientific graphics this is, in at least some
sense, a very good thing, because what cairo is doing amounts to drawing
the line closer to its true location. Two values that differ by less
than a pixel will actually look slightly different instead of getting
hammered into the same pixel row. For graphing, this is a big win, at
least for me.

-Jeff