Initialize backward refs with the pixel count

src/ImageSharp/Formats/Webp/Lossless/Vp8LBackwardRefs.cs

@@ -7,7 +7,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
 {
     internal class Vp8LBackwardRefs
     {
-        public Vp8LBackwardRefs() => this.Refs = new List<PixOrCopy>();
+        public Vp8LBackwardRefs(int pixels) => this.Refs = new List<PixOrCopy>(pixels);
 
         /// <summary>
         /// Gets or sets the common block-size.

src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs

@@ -130,7 +130,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
             int refsBlockSize = ((pixelCount - 1) / MaxRefsBlockPerImage) + 1;
             for (int i = 0; i < this.Refs.Length; i++)
             {
-                this.Refs[i] = new Vp8LBackwardRefs
+                this.Refs[i] = new Vp8LBackwardRefs(pixelCount)
                 {
                     BlockSize = refsBlockSize < MinBlockSize ? MinBlockSize : refsBlockSize
                 };