Add concave polygon algorithm using stencil buffer

[bvssvni]

Can be added to Polygon.

From nical's comment on IRC:

basically you take a point A inside of the shape and each triangle is made of P(i), P(i+1) and A. In the stencil buffer you alternate between 0 and and 1, and the pixels that are inside the shape are 1, while the ones outside are 0.