RoundRect::drawShadow(), blur が大きいときの描画の乱れを修正

[Reputeless]

# include <Siv3D.hpp> // Siv3D v0.6.13

void Main()
{
    Scene::SetBackground(ColorF{ 1.0 });

    while (System::Update())
    {
        {
            const RoundRect roundRect{ 50, 100, 300, 100, 5 };
            roundRect.rect.drawShadow(Vec2{ 0, 0 }, 5);
        }

        {
            const RoundRect roundRect{ 400, 100, 300, 100, 5 };
            roundRect.drawShadow(Vec2{ 0, 0 }, 20);
        }
    }
}