Add test for decoding only the first frame

tests/ImageSharp.Tests/Formats/Tiff/TiffDecoderTests.cs

@@ -4,6 +4,7 @@
 // ReSharper disable InconsistentNaming
 using System;
 using System.IO;
+using SixLabors.ImageSharp.Formats.Tiff;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
@@ -365,6 +366,17 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         public void TiffDecoder_CanDecode_PackBitsCompressed<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 
+        [Theory]
+        [WithFile(MultiFrameMipMap, PixelTypes.Rgba32)]
+        public void CanDecodeJustOneFrame<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new TiffDecoder() { DecodingMode = FrameDecodingMode.First }))
+            {
+                Assert.Equal(1, image.Frames.Count);
+            }
+        }
+
         [Theory]
         [WithFile(RgbJpegCompressed, PixelTypes.Rgba32)]
         [WithFile(RgbWithStripsJpegCompressed, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -867,6 +867,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string MultiframeDeflateWithPreview = "Tiff/multipage_deflate_withPreview.tiff";
             public const string MultiframeDifferentSize = "Tiff/multipage_differentSize.tiff";
             public const string MultiframeDifferentVariants = "Tiff/multipage_differentVariants.tiff";
+            public const string MultiFrameMipMap = "Tiff/SKC1H3.tiff";
 
             public const string LittleEndianByteOrder = "Tiff/little_endian.tiff";
 

tests/Images/Input/Tiff/SKC1H3.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:938bbf1c0f8bdbea0c632bb8d51c1150f757f88b3779d7fa18c296a3a3f61e9b
+size 13720193