Fix for the blur effect when the direction is set to .right and .up (reversed)

[alpennec]

Hello @TimOliver,

When using blurStartingInset for my blurs, I noticed an issue in your library when using the .right and .up directions for the blur.

The blur doesn't seem correct for the .right and .up directions as you can see on this video using the original code:

BlurUIKit_bug_720.mov

Here is another video with the code from this PR that fixes the issue:

BlurUIKit_fix_720.mov

Regards,
Axel

PS: please find below the SwiftUI code I used for the preview (not included in this PR).

//
//  SwiftUIExampleView.swift
//  BlurUIKitExample
//
//  Created by Tim Oliver on 10/2/2026.
//

import SwiftUI

struct SwiftUIExampleView: View {

    private let atlasImageSize = CGSize(width: 0.499, height: 0.249)
    private let fraction: CGFloat = 0.9
    
    var body: some View {
        GeometryReader { geometry in
            ScrollView {
                VStack(spacing: 16) {
                    ForEach(0..<8, id: \.self) { index in
                        atlasImage(index: index, width: geometry.size.width - 32)
                    }
                }
                .padding(.horizontal, 16)
                .padding(.top, geometry.safeAreaInsets.top + 8)
                .padding(.bottom, 16)
            }
            .ignoresSafeArea(edges: .top)
            .overlay(alignment: .top) {
                VariableBlur(direction: .down)
                    .blurStartingInset(.relative(fraction: fraction))
                    .ignoresSafeArea(edges: .top)
                    .frame(height: 100)
                    .border(.red)
            }
            .overlay {
                VStack {
                    VariableBlur(direction: .left)
                        .blurStartingInset(.relative(fraction: fraction))
                    
                    VariableBlur(direction: .right)
                        .blurStartingInset(.relative(fraction: fraction))
                }
                .frame(height: 200)
                .border(.red)
                .padding(.horizontal)
            }
            .overlay(alignment: .bottom) {
                    VariableBlur(direction: .up)
                        .blurStartingInset(.relative(fraction: fraction))
                        .ignoresSafeArea(edges: .bottom)
                        .frame(height: 100)
                        .border(.red)
            }
        }
    }

    @ViewBuilder
    private func atlasImage(index: Int, width: CGFloat) -> some View {
        let height = width / 1.777
        let fullWidth = width / atlasImageSize.width
        let fullHeight = height / atlasImageSize.height
        let originX = CGFloat(index % 2) * 0.5 * fullWidth
        let originY = CGFloat(index / 2) * 0.25 * fullHeight

        Image("AppleParkAtlas")
            .resizable()
            .frame(width: fullWidth, height: fullHeight)
            .offset(x: -originX, y: -originY)
            .frame(width: width, height: height, alignment: .topLeading)
            .clipShape(RoundedRectangle(cornerRadius: width * 0.1, style: .continuous))
    }
}

#Preview {
    SwiftUIExampleView()
}