Add tests for Orange4x4 image

ImageSharp.sln

@@ -671,6 +671,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Heif", "Heif", "{BA5D603A-C
 		tests\Images\Input\Heif\IMG-20230508-0053.hif = tests\Images\Input\Heif\IMG-20230508-0053.hif
 		tests\Images\Input\Heif\Irvine_CA.avif = tests\Images\Input\Heif\Irvine_CA.avif
 		tests\Images\Input\Heif\jpeg444_xnconvert.avif = tests\Images\Input\Heif\jpeg444_xnconvert.avif
+		tests\Images\Input\Heif\Orange4x4.avif = tests\Images\Input\Heif\Orange4x4.avif
 	EndProjectSection
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Icon", "Icon", "{95E45DDE-A67D-48AD-BBA8-5FAA151B860D}"

src/ImageSharp/Formats/Heif/Av1/Av1BitStreamReader.cs

@@ -150,8 +150,7 @@ internal ref struct Av1BitStreamReader
     {
         DebugGuard.IsTrue(Av1Math.Modulus8(this.BitPosition) == 0, "Symbol reading needs to start on byte boundary.");
         int bytesRead = Av1Math.DivideBy8Floor(this.BitPosition);
-        int spanLength = tileDataSize - bytesRead;
-        Span<byte> span = this.data.Slice(bytesRead, spanLength);
+        Span<byte> span = this.data.Slice(bytesRead, tileDataSize);
         this.Skip(tileDataSize << 3);
         return span;
     }

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1TilingTests.cs

@@ -10,24 +10,26 @@ namespace SixLabors.ImageSharp.Tests.Formats.Heif.Av1;
 [Trait("Format", "Avif")]
 public class Av1TilingTests
 {
-    // [Theory]
-    [InlineData(TestImages.Heif.XnConvert, 0x010E, 0x03CC, 18, 0x03CC - 18)]
-    public void ReadFirstTile(string filename, int headerOffset, int headerSize, int tileOffset, int tileSize)
+    [Theory]
+    /*[InlineData(TestImages.Heif.XnConvert, 0x010E, 0x03CC, 18)]*/
+    [InlineData(TestImages.Heif.Orange4x4, 0x010E, 0x001d, 21)]
+    public void ReadFirstTile(string filename, int dataOffset, int dataSize, int tileOffset)
     {
         // Assign
         string filePath = Path.Combine(TestEnvironment.InputImagesDirectoryFullPath, filename);
         byte[] content = File.ReadAllBytes(filePath);
-        Span<byte> headerSpan = content.AsSpan(headerOffset, headerSize);
-        Span<byte> tileSpan = content.AsSpan(tileOffset, tileSize);
+        Span<byte> headerSpan = content.AsSpan(dataOffset, dataSize);
+        Span<byte> tileSpan = content.AsSpan(tileOffset, dataSize - tileOffset);
         Av1BitStreamReader reader = new(headerSpan);
         IAv1TileDecoder stub = new Av1TileDecoderStub();
         ObuReader obuReader = new();
-        obuReader.ReadAll(ref reader, headerSize, stub);
+        obuReader.ReadAll(ref reader, dataSize, stub);
         Av1TileDecoder decoder = new(obuReader.SequenceHeader, obuReader.FrameHeader);
 
         // Act
         decoder.DecodeTile(tileSpan, 0);
 
         // Assert
+        Assert.Equal(dataSize * 8, reader.BitPosition);
     }
 }

tests/ImageSharp.Tests/Formats/Heif/Av1/ObuFrameHeaderTests.cs

@@ -25,6 +25,7 @@ public class ObuFrameHeaderTests
     // [InlineData(TestImages.Heif.IrvineAvif, 0x0102, 0x000D)]
     // [InlineData(TestImages.Heif.IrvineAvif, 0x0198, 0x6BD1)]
     [InlineData(TestImages.Heif.XnConvert, 0x010E, 0x03CC)]
+    [InlineData(TestImages.Heif.Orange4x4, 0x010E, 0x001d)]
     public void ReadFrameHeader(string filename, int fileOffset, int blockSize)
     {
         // Assign

tests/ImageSharp.Tests/TestImages.cs

@@ -1144,6 +1144,9 @@ public static class TestImages
         public const string IrvineAvif = "Heif/Irvine_CA.avif";
 
         public const string XnConvert = "Heif/jpeg444_xnconvert.avif";
+
+        // Extremely small image, 4x4 pixels with a single solid color.
+        public const string Orange4x4 = "Heif/Orange4x4.avif";
     }
 
     public static class Ico