An angular gradient.
An angular gradient is also known as a "conic" gradient. This gradient
applies the color function as the angle changes, relative to a center
point and defined start and end angles. If `endAngle - startAngle > 2π`,
the gradient only draws the last complete turn. If
`endAngle - startAngle < 2π`, the gradient fills the missing area with
the colors defined by gradient locations one and zero, transitioning
between the two halfway across the missing area. The gradient maps the
unit space center point into the bounding rectangle of each shape filled
with the gradient.
When using an angular gradient as a shape style, you can also use
``ShapeStyle/angularGradient(_:center:startAngle:endAngle:)``,
``ShapeStyle/conicGradient(_:center:angle:)``, or similar methods.