Generates vertex coordinates for a circle given its centre coordinates and radius.
circleVertices(xc, yc, radius, npoints = 25)Circle centre X ordinate.
Circle centre Y ordinate.
Circle radius.
Number of distinct vertices required.
A 2-column matrix of X and Y values. The final row is a copy
of the first row to create a closed polygon, so the matrix has
npoints + 1 rows.