Fix issue calculating the stripIndex for planar tiff's

src/ImageSharp/Formats/Tiff/TiffDecoderCore.cs

@@ -272,11 +272,11 @@ namespace SixLabors.ImageSharp.Formats.Tiff
                 {
                     int stripHeight = i < stripsPerPlane - 1 || frame.Height % rowsPerStrip == 0 ? rowsPerStrip : frame.Height % rowsPerStrip;
 
+                    int stripIndex = i;
                     for (int planeIndex = 0; planeIndex < stripsPerPixel; planeIndex++)
                     {
-                        int stripIndex = (i * stripsPerPixel) + planeIndex;
-
                         decompressor.Decompress(this.inputStream, (uint)stripOffsets[stripIndex], (uint)stripByteCounts[stripIndex], stripBuffers[planeIndex].GetSpan());
+                        stripIndex += stripsPerPlane;
                     }
 
                     colorDecoder.Decode(stripBuffers, pixels, 0, rowsPerStrip * i, frame.Width, stripHeight);

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

@@ -84,6 +84,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Tiff
         public void TiffDecoder_CanDecode_Uncompressed<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
 
+        [Theory]
+        [WithFile(FlowerRgb888Planar6Strips, PixelTypes.Rgba32)]
+        [WithFile(FlowerRgb888Planar15Strips, PixelTypes.Rgba32)]
+        public void TiffDecoder_Planar<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel> => TestTiffDecoder(provider);
+
         [Theory]
         [WithFile(Calliphora_PaletteUncompressed, PixelTypes.Rgba32)]
         [WithFile(PaletteDeflateMultistrip, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -574,6 +574,8 @@ namespace SixLabors.ImageSharp.Tests
             public const string FlowerRgb444Planar = "Tiff/flower-rgb-planar-04.tiff";
             public const string FlowerRgb222Contiguous = "Tiff/flower-rgb-contig-02.tiff";
             public const string FlowerRgb222Planar = "Tiff/flower-rgb-planar-02.tiff";
+            public const string FlowerRgb888Planar6Strips = "Tiff/flower-rgb-planar-08-6strips.tiff";
+            public const string FlowerRgb888Planar15Strips = "Tiff/flower-rgb-planar-08-15strips.tiff";
             public const string Flower2BitGray = "Tiff/flower-minisblack-02.tiff";
             public const string Flower2BitPalette = "Tiff/flower-palette-02.tiff";
             public const string Flower4BitPalette = "Tiff/flower-palette-04.tiff";

tests/Images/Input/Tiff/flower-rgb-planar-08-15strips.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:a49cf47fdf2ea43e5cb5a473523e50222fb13ff6a66bda2e4bdd5796f66140d8
+size 9770

tests/Images/Input/Tiff/flower-rgb-planar-08-6strips.tiff

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:a49cf47fdf2ea43e5cb5a473523e50222fb13ff6a66bda2e4bdd5796f66140d8
+size 9770