Adding qoi identification

I'm going to do some tests before the pull request and add all the functions

src/ImageSharp/Formats/Qoi/MetadataExtensions.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Qoi;
+using SixLabors.ImageSharp.Metadata;
+
+namespace SixLabors.ImageSharp;
+
+/// <summary>
+/// Extension methods for the <see cref="ImageMetadata"/> type.
+/// </summary>
+public static partial class MetadataExtensions
+{
+    /// <summary>
+    /// Gets the qoi format specific metadata for the image.
+    /// </summary>
+    /// <param name="metadata">The metadata this method extends.</param>
+    /// <returns>The <see cref="QoiMetadata"/>.</returns>
+    public static QoiMetadata GetQoiMetadata(this ImageMetadata metadata) => metadata.GetFormatMetadata(QoiFormat.Instance);
+}

src/ImageSharp/Formats/Qoi/QoiDecoder.cs

@@ -0,0 +1,27 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+internal class QoiDecoder : ImageDecoder
+{
+    protected override Image<TPixel> Decode<TPixel>(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
+    {
+        Guard.NotNull(options, nameof(options));
+        Guard.NotNull(stream, nameof(stream));
+        throw new NotImplementedException();
+    }
+
+    protected override Image Decode(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
+    {
+        Guard.NotNull(options, nameof(options));
+        Guard.NotNull(stream, nameof(stream));
+        throw new NotImplementedException();
+    }
+
+    protected override ImageInfo Identify(DecoderOptions options, Stream stream, CancellationToken cancellationToken)
+    {
+        Guard.NotNull(options, nameof(options));
+        Guard.NotNull(stream, nameof(stream));
+        throw new NotImplementedException();
+    }
+}

src/ImageSharp/Formats/Qoi/QoiDecoderCore.cs

@@ -0,0 +1,93 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.IO;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.Metadata;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+internal class QoiDecoderCore : IImageDecoderInternals
+{
+    /// <summary>
+    ///     The global configuration.
+    /// </summary>
+    private readonly Configuration configuration;
+
+    /// <summary>
+    ///     Used the manage memory allocations.
+    /// </summary>
+    private readonly MemoryAllocator memoryAllocator;
+
+    /// <summary>
+    ///     Gets or sets a value indicating whether the metadata should be ignored when the image is being decoded.
+    /// </summary>
+    private readonly bool skipMetadata;
+
+    /// <summary>
+    ///     The QOI header.
+    /// </summary>
+    private QoiHeader header;
+
+    public QoiDecoderCore(DecoderOptions options)
+    {
+        this.Options = options;
+        this.configuration = options.Configuration;
+        this.skipMetadata = options.SkipMetadata;
+        this.memoryAllocator = this.configuration.MemoryAllocator;
+    }
+
+    public DecoderOptions Options { get; }
+
+    public Size Dimensions { get; }
+
+    public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, CancellationToken cancellationToken)
+        where TPixel : unmanaged, IPixel<TPixel> => throw new NotImplementedException();
+
+    public ImageInfo Identify(BufferedReadStream stream, CancellationToken cancellationToken)
+    {
+        ImageMetadata metadata = new();
+
+        byte[] widthBytes, heightBytes;
+        byte[] magicBytes = widthBytes = heightBytes = Array.Empty<byte>();
+
+        // Read magic bytes
+        int read = stream.Read(magicBytes, 0, 4);
+        if (read != 4 || !magicBytes.Equals(QoiConstants.Magic.ToArray()))
+        {
+            throw new InvalidImageContentException("The image is not a QOI image");
+        }
+
+        // If it's a qoi image, read the rest of properties
+        read = stream.Read(widthBytes, 0, 4);
+        if (read != 4)
+        {
+            throw new InvalidImageContentException("The image is not a QOI image");
+        }
+
+        read = stream.Read(heightBytes, 0, 4);
+        if (read != 4)
+        {
+            throw new InvalidImageContentException("The image is not a QOI image");
+        }
+
+        Size size = new(BitConverter.ToInt32(widthBytes), BitConverter.ToInt32(heightBytes));
+
+        int channels = stream.ReadByte();
+        if (channels == -1)
+        {
+            throw new InvalidImageContentException("The image is not a QOI image");
+        }
+
+        PixelTypeInfo pixelType = new(8 * channels);
+
+        int colorSpace = stream.ReadByte();
+        if (colorSpace == -1)
+        {
+            throw new InvalidImageContentException("The image is not a QOI image");
+        }
+
+        return new ImageInfo(pixelType, size, metadata);
+    }
+}

src/ImageSharp/Formats/Qoi/QoiFormat.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Png;
+
+namespace SixLabors.ImageSharp.Formats.Qoi;
+
+/// <summary>
+/// Registers the image encoders, decoders and mime type detectors for the qoi format.
+/// </summary>
+public sealed class QoiFormat : IImageFormat<QoiMetadata>
+{
+    private QoiFormat()
+    { }
+
+    /// <summary>
+    /// Gets the shared instance.
+    /// </summary>
+    public static QoiFormat Instance { get; } = new QoiFormat();
+
+    /// <inheritdoc/>
+    public QoiMetadata CreateDefaultFormatMetadata() => new();
+
+    /// <inheritdoc/>
+    public string Name => "QOI";
+
+    /// <inheritdoc/>
+    public string DefaultMimeType => "image/qoi";
+
+    /// <inheritdoc/>
+    public IEnumerable<string> MimeTypes => QoiConstants.MimeTypes;
+
+    /// <inheritdoc/>
+    public IEnumerable<string> FileExtensions => QoiConstants.FileExtensions;
+}

src/ImageSharp/Formats/Qoi/QoiMetadata.cs

@@ -1,9 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-using System.Text;
-using SixLabors.ImageSharp.PixelFormats;
-
 namespace SixLabors.ImageSharp.Formats.Qoi;
 
 /// <summary>