Test Screen ‘FillPoly’ for speed and correctness.
Screen ‘FillPoly’ renders both convex and concave polygons, but treats
them differently; Convex polygons it renders quickly using GL_POLYGON.
Concave polygons it renders slowly using GLU tesselators.
FillPolyTest draws a “Butterfly” concave polygon to test if FillPolly
correctly detects and renders concave polygons. If FillPoly failed to
detect concavity, it would use GL_POLYGON, which renders concave polygons