Make the rendering of the stroke outline *almost* transparent when in preview mode.
If you try to make it fully transparent then you will find that it is fully opauqe instead.
So it is now at 1/255 = 0.4% opaque, which I can't see when rendered even if the colors
are bright green or red.
Merge pull request #1571 from monkeyiq/2014/aug/1217

Make the rendering of the stroke outline *almost* transparent when in preview mode.