Use RootFrame for validation

src/ImageSharp/Image/ImageFrameCollection.cs

@@ -17,14 +17,6 @@ namespace SixLabors.ImageSharp
         where TPixel : struct, IPixel<TPixel>
     {
         private readonly IList<ImageFrame<TPixel>> frames = new List<ImageFrame<TPixel>>();
-        private readonly int parentWidth;
-        private readonly int parentHeight;
-
-        internal ImageFrameCollection(Image<TPixel> parent)
-        {
-            this.parentWidth = parent.Width;
-            this.parentHeight = parent.Height;
-        }
 
         /// <summary>
         /// Gets the count.
@@ -103,7 +95,7 @@ namespace SixLabors.ImageSharp
         {
             if (this.Count != 0)
             {
-                if (this.parentWidth != frame.Width || this.parentHeight != frame.Height)
+                if (this.RootFrame.Width != frame.Width || this.RootFrame.Height != frame.Height)
                 {
                     throw new ArgumentException("Frame must have the same dimensions as the image", nameof(frame));
                 }

src/ImageSharp/Image/Image{TPixel}.cs

@@ -1,22 +1,14 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
-using System.Collections;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.IO;
 using System.Linq;
-using System.Numerics;
-using System.Text;
-using System.Threading.Tasks;
-using SixLabors.ImageSharp.Advanced;
+
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.ImageSharp.Processing;
-using SixLabors.Primitives;
 
 namespace SixLabors.ImageSharp
 {
@@ -81,7 +73,7 @@ namespace SixLabors.ImageSharp
             this.ImageConfiguration = configuration ?? Configuration.Default;
             this.MetaData = metadata ?? new ImageMetaData();
 
-            this.Frames = new ImageFrameCollection<TPixel>(this);
+            this.Frames = new ImageFrameCollection<TPixel>();
 
             if (frames != null)
             {