[cairo] Survey of polygon rasterization techniques

[Kiia Kallio]

Flash can have self-intersecting shapes, as it supports morphing. Even if the
source shapes don't have self-intersection, a morph between two shapes can
result in it. Self-intersection is resolved using even-odd fill rule. (It's easy to
test this if you have flash tools for creating some files.)

Rasterizers with analytical coverage calculation can't render Flash properly
unless they resolve the coverage for the whole set of polygons at each pixel.
AGG's compound rasterizer does this within one flash shape (consisting of several
polygons), and thus eliminates "seams" from inside the shape, but it still would
leave a seam at the shared edge between two neighbouring shapes.

Kiia