Fix issue with packed bits and bi color tiffs

src/ImageSharp/Formats/Tiff/README.md

@@ -42,7 +42,7 @@
 |---------------------------|:-----:|:-----:|--------------------------|
 |None                       |   Y   |   Y   |                          |
 |Ccitt1D                    |   Y   |   Y   |                          |
-|PackBits                   |       |   Y   |                          |
+|PackBits                   |   Y   |   Y   |                          |
 |CcittGroup3Fax             |   Y   |   Y   |                          |
 |CcittGroup4Fax             |       |       |                          |
 |Lzw                        |       |   Y   | Based on ImageSharp GIF LZW implementation - this code could be modified to be (i) shared, or (ii) optimised for each case |

src/ImageSharp/Formats/Tiff/Utils/TiffWriter.cs

@@ -591,7 +591,7 @@ namespace SixLabors.ImageSharp.Formats.Tiff.Utils
                 }
 
                 var size = PackBitsWriter.PackBits(outputRow, compressedRowSpan);
-                this.output.Write(compressedRowSpan);
+                this.output.Write(compressedRowSpan.Slice(0, size));
                 bytesWritten += size;
 
                 outputRow.Clear();