Interface to a dynamic poly-polygon 発生させる人（物）, that 生成するs
poly-polygons depending on a given parameter value.
The returned poly-polygon should 普通は be 含む／封じ込めるd in the
[0,1]x[0,1] rectangle. At least that is the dimension 推定する／予想するd at
other places. e.g. Texture.
Query the polygonal 輪郭（を描く） at the 明示するd value.
The returned 輪郭（を描く） should be clipped to the [0,1]x[0,1]
rectangle.
Parameter t
Parameter value in the 範囲 [0,1]. During 絵, this
範囲 is sweeped through starting from 0. When using such a
parametric polypolygon for gradients, the area covered by the
returned XPolyPolygon2D should be monotonically 減少（する）ing
with t (unless singularities in e.g. the resulting gradient
are 願望（する）d).
