Fix decoding issues

- Lossless: Check if transforms are present on dispose
- Lossy: Set filterinfo properly

src/ImageSharp/Formats/WebP/Vp8FilterInfo.cs

@@ -6,8 +6,27 @@ namespace SixLabors.ImageSharp.Formats.WebP
     /// <summary>
     /// Filter information.
     /// </summary>
-    internal class Vp8FilterInfo
+    internal class Vp8FilterInfo : IDeepCloneable
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Vp8FilterInfo"/> class.
+        /// </summary>
+        public Vp8FilterInfo()
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Vp8FilterInfo"/> class.
+        /// </summary>
+        /// <param name="other">The filter info to create an instance from.</param>
+        public Vp8FilterInfo(Vp8FilterInfo other)
+        {
+            this.Limit = other.Limit;
+            this.HighEdgeVarianceThreshold = other.HighEdgeVarianceThreshold;
+            this.InnerLevel = other.InnerLevel;
+            this.UseInnerFiltering = other.UseInnerFiltering;
+        }
+
         /// <summary>
         /// Gets or sets the filter limit in [3..189], or 0 if no filtering.
         /// </summary>
@@ -28,5 +47,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// Gets or sets the high edge variance threshold in [0..2].
         /// </summary>
         public byte HighEdgeVarianceThreshold { get; set; }
+
+        /// <inheritdoc/>
+        public IDeepCloneable DeepClone() => new Vp8FilterInfo(this);
     }
 }

src/ImageSharp/Formats/WebP/Vp8LDecoder.cs

@@ -57,12 +57,15 @@ namespace SixLabors.ImageSharp.Formats.WebP
         public void Dispose()
         {
             this.Pixels.Dispose();
-            foreach (Vp8LTransform transform in this.Transforms)
+            this.Metadata?.HuffmanImage?.Dispose();
+
+            if (this.Transforms != null)
             {
-                transform.Data?.Dispose();
+                foreach (Vp8LTransform transform in this.Transforms)
+                {
+                    transform.Data?.Dispose();
+                }
             }
-
-            this.Metadata?.HuffmanImage?.Dispose();
         }
     }
 }

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -79,9 +79,11 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
             if (info.Features?.Alpha is true)
             {
-                var alphaDecoder = new AlphaDecoder(width, height, info.Features.AlphaData, info.Features.AlphaChunkHeader, this.memoryAllocator);
-                alphaDecoder.Decode();
-                this.DecodePixelValues(width, height, decoder.Pixels, pixels, alphaDecoder.Alpha);
+                using (var alphaDecoder = new AlphaDecoder(width, height, info.Features.AlphaData, info.Features.AlphaChunkHeader, this.memoryAllocator))
+                {
+                    alphaDecoder.Decode();
+                    this.DecodePixelValues(width, height, decoder.Pixels, pixels, alphaDecoder.Alpha);
+                }
             }
             else
             {
@@ -100,7 +102,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 hasAlpha = true;
                 alphaSpan = alpha.Memory.Span;
             }
-            
+
             for (int y = 0; y < height; y++)
             {
                 Span<TPixel> pixelRow = pixels.GetRowSpan(y);
@@ -826,7 +828,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
             // Store filter info.
             if (dec.Filter != LoopFilter.None)
             {
-                dec.FilterInfo[dec.MbX] = dec.FilterStrength[blockData.Segment, blockData.IsI4x4 ? 1 : 0];
+                Vp8FilterInfo precomputedFilterInfo = dec.FilterStrength[blockData.Segment, blockData.IsI4x4 ? 1 : 0];
+                dec.FilterInfo[dec.MbX] = (Vp8FilterInfo)precomputedFilterInfo.DeepClone();
                 dec.FilterInfo[dec.MbX].UseInnerFiltering |= (byte)(skip is 0 ? 1 : 0);
             }
         }