Support even-odd winding rule for fills

[systemed]

At present Prawn fills using the 'f' and 'b' operators. For nested shapes, this will use the default PDF "non-zero winding rule" to determine what's inside the fill and what isn't.

For some purposes, the alternative "even-odd" rule is useful. In particular, where you have one shape inside another (a hole), the even-odd rule will ensure the hole isn't filled without having to worry about the clockwiseness of each shape.

It would be good if Prawn provided an option to use the even-odd rule.

The only difference in PDF output is that you use the 'f_' and 'b_' operators rather than 'f' and 'b'. (Section 4.4.2 of the PDF spec if it helps!) I can of course use pdf.add_content("f*") for the interim, but an option would be neater. :)

[bradediger]

Good idea. I've added this via an option to the fill / fill_and_stroke operators: just pass in :fill_rule => :even_odd.

I also added a page to the manual with a visual example of the difference between the two rules.

Thanks!