Enable dedup for png

Author: JimBobSquarePants

src/ImageSharp/Formats/AnimationUtilities.cs

@@ -85,11 +85,12 @@ public static (bool Difference, Rectangle Bounds) DeDuplicatePixels<TPixel>(
                     int m = Avx2.MoveMask(neq.AsByte());
                     if (m != 0)
                     {
-                        // If is diff is found, the left side is marked by the min of previously found left side and the diff position.
-                        // The right is the max of the previously found right side and the diff position + 1.
-                        int diff = (int)(i + (uint)(BitOperations.TrailingZeroCount(m) / size));
-                        left = Math.Min(left, diff);
-                        right = Math.Max(right, diff + 1);
+                        // If is diff is found, the left side is marked by the min of previously found left side and the start position.
+                        // The right is the max of the previously found right side and the end position.
+                        int start = i + (BitOperations.TrailingZeroCount(m) / size);
+                        int end = i + (2 - (BitOperations.LeadingZeroCount((uint)m) / size));
+                        left = Math.Min(left, start);
+                        right = Math.Max(right, end);
                         hasRowDiff = true;
                         hasDiff = true;
                     }

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -797,6 +797,8 @@ private void ReadLogicalScreenDescriptorAndGlobalColorTable(BufferedReadStream s
                 this.gifMetadata.GlobalColorTable = colorTable;
             }
         }
+
+        this.gifMetadata.BackgroundColorIndex = this.logicalScreenDescriptor.BackgroundColorIndex;
     }
 
     private unsafe struct ScratchBuffer

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -245,8 +245,6 @@ private void EncodeAdditionalFrames<TPixel>(
         ImageFrame<TPixel> previousFrame = image.Frames.RootFrame;
 
         // This frame is reused to store de-duplicated pixel buffers.
-        // This is more expensive memory-wise than de-duplicating indexed buffer but allows us to deduplicate
-        // frames using both local and global palettes.
         using ImageFrame<TPixel> encodingFrame = new(previousFrame.Configuration, previousFrame.Size());
 
         for (int i = 1; i < image.Frames.Count; i++)

src/ImageSharp/Formats/Gif/MetadataExtensions.cs

@@ -83,7 +83,7 @@ internal static AnimatedImageFrameMetadata ToAnimatedImageFrameMetadata(this Gif
             ColorTableMode = source.ColorTableMode == GifColorTableMode.Global ? FrameColorTableMode.Global : FrameColorTableMode.Local,
             Duration = TimeSpan.FromMilliseconds(source.FrameDelay * 10),
             DisposalMode = GetMode(source.DisposalMethod),
-            BlendMode = FrameBlendMode.Source,
+            BlendMode = FrameBlendMode.Over,
         };
 
     private static FrameDisposalMode GetMode(GifDisposalMethod method) => method switch