Metal每日分享，调整透明度滤镜效果

// 透明度滤镜
let filter = C7Opacity.init(opacity: 0.75)
// 方案1:
ImageView.image = try? BoxxIO(element: originImage, filters: [filter, filter2, filter3]).output()
// 方案2:
ImageView.image = originImage.filtering(filter, filter2, filter3)
// 方案3:
ImageView.image = originImage ->> filter ->> filter2 ->> filter3

/// 透明度调整，核心就是改变`alpha`
public struct C7Opacity: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: 0.0, max: 1.0, value: 1.0)
    /// Change the opacity of an image, from 0.0 to 1.0, with a default of 1.0
    @ZeroOneRange public var opacity: Float = range.value
    public var modifier: Modifier {
        return .compute(kernel: "C7Opacity")
    }
    public var factors: [Float] {
        return [opacity]
    }
    public init(opacity: Float = range.value) {
        self.opacity = opacity
    }
}

kernel void C7Opacity(texture2d<half, access::write> outputTexture [[texture(0)]],
                      texture2d<half, access::read> inputTexture [[texture(1)]],
                      device float *opacity [[buffer(0)]],
                      uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    const half4 outColor = half4(inColor.rgb, half(*opacity) * inColor.a);
    outputTexture.write(outColor, grid);
}