Creates an empty GPolygon. The object's x and y coordinates can also be
specified — if left unspecified, defaults to 0, 0.
Methods
function GPolygon::addVertex(x, y)
Adds a vertex at (x, y) relative to the polygon's origin.
function GPolygon::addEdge(dx, dy)
Adds a vertex at wherever the last point was placed (or 0, 0 if
this is the first), shifted by dx and dy.
function GPolygon::addPolarEdge(r, theta)
Adds a vertex at wherever the last point was placed (or, 0, 0 if
this is the first), shifted by r units in the direction theta.
theta is measured in degrees counterclockwise from the +x axis.