Calculates an elliptical arc, and sets the vertices in the destination
buffer to the calculated positions.
If C]thickness <= 0], then C]num_points] of points are
required in the destination, otherwise twice as many are needed.
The destination buffer should consist of regularly spaced (by distance
of C]stride] bytes) doublets of floats, corresponding to x and y
coordinates of the vertices.

I]Parameters:]

[bu]

dest - The destination buffer

[bu]

stride - Distance (in bytes) between starts of successive pairs of
coordinates

[bu]

cx, cy - Center of the arc

[bu]

rx, ry - Radii of the arc

[bu]

start_theta - The initial angle from which the arc is calculated

[bu]

delta_theta - Angular span of the arc (pass a negative number to switch
direction)