FilterEffectで複数のターゲットを処理できるようにする

[yuto-trd]

API案

public readonly struct Rect
{
    // X: NegativeInfinity, Y: NegativeInfinity, Width: PositiveInfinity, Height: PositiveInfinity
    public static readonly Rect Infinity;
    // X: NaN, Y: NaN, Width: NaN, Height: NaN
    public static readonly Rect Invalid;

    public bool IsInvalid { get; }
}

public class FilterEffectContext
{
    public Rect Bounds { get; }

    public void Custom<T>(T data, Action<T, FilterEffectCustomOperationContext> action);
}

public class FilterEffectCustomOperationContext
{
    public IList<EffectTarget> Targets { get; }
}