Fixing Qoi detector

-I had several errors about header size comparison and comparing byte arrays, but I've fixed them and tested the Identify function and it works perfect!

Author: LuisAlfredo92

src/ImageSharp/Formats/Qoi/QoiImageFormatDetector.cs

@@ -23,5 +23,5 @@ public bool TryDetectFormat(ReadOnlySpan<byte> header, [NotNullWhen(true)] out I
     }
 
     private bool IsSupportedFileFormat(ReadOnlySpan<byte> header)
-        => header.Length == this.HeaderSize && header[..4] == QoiConstants.Magic;
+        => header.Length >= this.HeaderSize && QoiConstants.Magic.SequenceEqual(header[..4]);
 }

tests/ImageSharp.Tests/Formats/Qoi/QoiDecoderTests.cs

@@ -24,5 +24,6 @@ public void Identify(string imagePath)
         ImageInfo imageInfo = Image.Identify(stream);
 
         Assert.NotNull(imageInfo);
+        Assert.Equal(imageInfo.Metadata.DecodedImageFormat, ImageSharp.Formats.Qoi.QoiFormat.Instance);
     }
 }