Function radial_gradient 

pub fn radial_gradient<'a>(
    (center, radius): (impl Into<Point>, scalar),
    gradient: &Gradient<'_>,
    local_matrix: impl Into<Option<&'a Matrix>>,
) -> Option<Shader>

Returns a shader that generates a radial gradient given the center and radius.

• center: The center of the circle for this gradient
• radius: Must be positive. The radius of the circle for this gradient
• gradient: Description of the colors and interpolation method
• local_matrix: Optional local matrix