LibWeb: Not following shadowbox border radius decreasing standard

[SuntzuDragon]

Student project from the University of Utah!
This formula $1 + (r-1)^3$ is not being used for box shadow corner radius modification, but it should be (see last paragraph in section on csswg).

Right now, it is simply being added:

auto spread_corner = [&](auto& corner) {
    if (corner) {
        corner.horizontal_radius += spread_distance.value();
        corner.vertical_radius += spread_distance.value();
    }
};

[SuntzuDragon]

Code like this should fix it:

auto spread_corner = [&](auto& corner) {
    if (corner) {
        auto spread_distance_value = spread_distance.value();
        // When radii < spread distance, modify spread distance in each direction per last paragraph of https://drafts.csswg.org/css-backgrounds/#shadow-shape
        auto horizontal_spread_distance = spread_distance_value;
        if (corner.horizontal_radius < spread_distance_value) {
            auto radius_spread_ratio = static_cast<float>(corner.horizontal_radius) / spread_distance_value;
            horizontal_spread_distance *= 1 + pow((radius_spread_ratio - 1), 3);
        }
        corner.horizontal_radius += round(horizontal_spread_distance);

        auto vertical_spread_distance = spread_distance_value;
        if (corner.vertical_radius < spread_distance_value) {
            auto radius_spread_ratio = static_cast<float>(corner.vertical_radius) / spread_distance_value;
            vertical_spread_distance *= 1 + pow((radius_spread_ratio - 1), 3);
        }
        corner.vertical_radius += round(vertical_spread_distance);
    }
};

I tested this against the example csswg gave:

For example, if the border radius is 10px and the spread distance is 20px (r = .5), the corner radius of the shadow shape will be 10px + 20px × (1 + (.5 - 1)3) = 27.5px rather than 30px.

And found the same results (rounded because CSSPixels use integers):

horizontal: 10
vertical: 10
spread_distance: 20
horizontal ratio: 0.5
horizontal pow -0.125
new horizontal spread: 17
vertical ratio: 0.5
vertical pow: -0.125
new vertical spread: 17
new horizontal: 27
new vertical: 27