Fix image data size in webpinfo for VP8: 10 bytes are already read during parsing the header

src/ImageSharp/Formats/WebP/WebPDecoderCore.cs

@@ -341,7 +341,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                        Height = height,
                        BitsPerPixel = features?.Alpha is true ? WebPBitsPerPixel.Pixel32 : WebPBitsPerPixel.Pixel24,
                        IsLossLess = false,
-                       ImageDataSize = dataSize,
+                       ImageDataSize = dataSize - 10, // 10 bytes are read here in the header already.
                        Features = features,
                        Vp8Profile = (sbyte)version
                    };

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -60,13 +60,6 @@ namespace SixLabors.ImageSharp.Formats.WebP
         }
     }
 
-    enum LoopFilter
-    {
-        Normal,
-        Simple,
-        None
-    }
-
     struct YUVPixel
     {
         public byte Y { get; }