Add ANI decoder support

src/ImageSharp/Formats/Ani/AniChunkType.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Ani;
+
+internal enum AniChunkType : uint
+{
+    /// <summary>
+    /// "anih"
+    /// </summary>
+    AniH = 0x68_69_6E_61,
+
+    /// <summary>
+    /// "seq "
+    /// </summary>
+    Seq = 0x20_71_65_73,
+
+    /// <summary>
+    /// "rate"
+    /// </summary>
+    Rate = 0x65_74_61_72,
+
+    /// <summary>
+    /// "LIST"
+    /// </summary>
+    List = 0x54_53_49_4C
+}
+
+/// <summary>
+/// ListType
+/// </summary>
+internal enum AniListType : uint
+{
+    /// <summary>
+    /// "INFO" (ListType)
+    /// </summary>
+    Info = 0x4F_46_4E_49,
+
+    /// <summary>
+    /// "fram"
+    /// </summary>
+    Fram = 0x6D_61_72_66
+}
+
+/// <summary>
+/// in "INFO"
+/// </summary>
+internal enum AniListInfoType : uint
+{
+    /// <summary>
+    /// "INAM"
+    /// </summary>
+    INam = 0x4D_41_4E_49,
+
+    /// <summary>
+    /// "IART"
+    /// </summary>
+    IArt = 0x54_52_41_49
+}
+
+/// <summary>
+/// in "Fram"
+/// </summary>
+internal enum AniListFrameType : uint
+{
+    /// <summary>
+    /// "icon"
+    /// </summary>
+    Icon = 0x6E_6F_63_69
+}

src/ImageSharp/Formats/Ani/AniConfigurationModule.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Ani;
+
+/// <summary>
+/// Registers the image encoders, decoders and mime type detectors for the Ico format.
+/// </summary>
+public sealed class AniConfigurationModule : IImageFormatConfigurationModule
+{
+    /// <inheritdoc/>
+    public void Configure(Configuration configuration)
+    {
+        // configuration.ImageFormatsManager.SetEncoder(AniFormat.Instance, new AniEncoder());
+        configuration.ImageFormatsManager.SetDecoder(AniFormat.Instance, AniDecoder.Instance);
+        configuration.ImageFormatsManager.AddImageFormatDetector(new AniImageFormatDetector());
+    }
+}

src/ImageSharp/Formats/Ani/AniConstants.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Ani;
+
+internal static class AniConstants
+{
+    /// <summary>
+    /// Gets the header bytes identifying an ani.
+    /// </summary>
+    public const uint AniFourCc = 0x41_43_4F_4E;
+
+    /// <summary>
+    /// The list of mime types that equate to an ani.
+    /// </summary>
+    public static readonly IEnumerable<string> MimeTypes = ["application/x-navi-animation"];
+
+    /// <summary>
+    /// The list of file extensions that equate to an ani.
+    /// </summary>
+    public static readonly IEnumerable<string> FileExtensions = ["ani"];
+
+    /// <summary>
+    /// Gets the header bytes identifying an ani.
+    /// </summary>
+    public static ReadOnlySpan<byte> AniFormTypeFourCc => "ACON"u8;
+}

src/ImageSharp/Formats/Ani/AniDecoder.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Ani;
+
+internal sealed class AniDecoder : ImageDecoder
+{
+    private AniDecoder()
+    {
+    }
+
+    /// <summary>
+    /// Gets the shared instance.
+    /// </summary>
+    public static AniDecoder Instance { get; } = new();
+
+    protected override Image<TPixel> Decode<TPixel>(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
+    {
+        Guard.NotNull(options, nameof(options));
+        Guard.NotNull(stream, nameof(stream));
+        Image<TPixel> image = new AniDecoderCore(options).Decode<TPixel>(options.Configuration, stream, cancellationToken);
+        ScaleToTargetSize(options, image);
+        return image;
+    }
+
+    protected override Image Decode(DecoderOptions options, Stream stream, CancellationToken cancellationToken) => this.Decode<Rgba32>(options, stream, cancellationToken);
+
+    protected override ImageInfo Identify(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
+    {
+        Guard.NotNull(options, nameof(options));
+        Guard.NotNull(stream, nameof(stream));
+        return new AniDecoderCore(options).Identify(options.Configuration, stream, cancellationToken);
+    }
+}