function polarToCartesian(polarcoords) {
    var cartesianCoords = [];
    var radius = polarcoords[0];
    var theta = polarcoords[1];
    var phi = polarcoords[2];

    var x = Math.abs(radius * Math.cos(phi * Math.PI / 180)) * Math.sin(theta * Math.PI / 180);
    var y = radius * Math.sin(phi * Math.PI / 180);
    var z = - (Math.abs(radius * Math.cos(phi * Math.PI / 180)) * Math.cos(theta * Math.PI / 180));
    cartesianCoords.push(x);
    cartesianCoords.push(y);
    cartesianCoords.push(z);
    return cartesianCoords;
}
Convert the given polar coords of the form [r, theta, phi] to cartesian
coordinates based on the default user location of (0, 0, 0) w/ viewing vector
of (0, 0, -1) and up vector of (0, 1, 0).

r - radius of the line
theta - angle to the right of the viewing vector
phi - angle up from the viewing vector