Interface to a dynamic poly-polygon generator, that generates
poly-polygons depending on a given parameter value.
The returned poly-polygon should normally be contained in the
[0,1]x[0,1] rectangle. At least that is the dimension expected at
other places. e.g. Texture.
Query the polygonal outline at the specified value.
The returned outline should be clipped to the [0,1]x[0,1]
rectangle.
Parameter t
Parameter value in the range [0,1]. During painting, this
range is sweeped through starting from 0. When using such a
parametric polypolygon for gradients, the area covered by the
returned XPolyPolygon2D should be monotonically decreasing
with t (unless singularities in e.g. the resulting gradient
are desired).
