don't render 0-width lines to PDF

[robx]

This adds a test that shows that currently a 0-width line ends up in the PDF but not in the PNG.

This may depend on the PDF viewer, compare https://tex.stackexchange.com/questions/210125/tikz-zero-width-line-with-dashed-line. Width 0 is "1 device pixel" by spec.

And adds a change to omit width-0 lines from the PDF. (This seems to do just that, but I'm not really that sure of how the code in question works and whether something goes missing now.)

[robx]

This is intended to fix https://github.com/diagrams/diagrams-rasterific/issues/43

[Twinside]

Thanks, I'll upload new version to hackage tomorrow morning