rect and rectv support efficient specification of rectangles as two
corner points. Each rectangle command takes four arguments, organized either
as two consecutive pairs of (x, y) coordinates, or as two pointers to
arrays, each containing an (x, y) pair. The resulting rectangle is
defined in the z = 0 plane.
rect (Vertex2 x1 y1) (Vertex2 x2, y2) is exactly equivalent to the
following sequence:
renderPrimitive Polygon $ do
vertex (Vertex2 x1 y1)
vertex (Vertex2 x2 y1)
vertex (Vertex2 x2 y2)
vertex (Vertex2 x1 y2)
Note that if the second vertex is above and to the right of the first vertex,
the rectangle is constructed with a counterclockwise winding.
