Line colors don't blend the same on Mac/Linux as they do on Windows

[astrofrog]

Line colors blend properly on Windows and seem nicely antialiased

(the selected constellation is supposed to be blue)

But on Mac/Linux the blue doesn't blend with the red:

I'm not sure if there's much we can do about it, but this is causing the AppVeyor tests to fail. I'm actually not sure which rendering is the intended one.