Added custom RacetrackQuad class which is used to draw the road
polygons. Tried the Render2D graphics extension at first but it turns
out that it's very slow. I think it wasn't made to draw thousands of
shapes 60 times per second.

Unlike the Quad2D class, The RacetrackQuad takes four point coords as
arguments so that the four sides of the polygon can be different in
length.