Add explicit bit values to BitsPerSample

src/ImageSharp/Formats/Tiff/TiffBitsPerSample.cs

@@ -11,26 +11,26 @@ namespace SixLabors.ImageSharp.Formats.Tiff
         /// <summary>
         /// The Bits per samples is not known.
         /// </summary>
-        Unknown,
+        Unknown = 0,
 
         /// <summary>
         /// One bit per sample for bicolor images.
         /// </summary>
-        Bit1,
+        Bit1 = 1,
 
         /// <summary>
         /// Four bits per sample for grayscale images with 16 different levels of gray or paletted images with a palette of 16 colors.
         /// </summary>
-        Bit4,
+        Bit4 = 4,
 
         /// <summary>
         /// Eight bits per sample for grayscale images with 256 different levels of gray or paletted images with a palette of 256 colors.
         /// </summary>
-        Bit8,
+        Bit8 = 8,
 
         /// <summary>
         /// 24 bits per sample, each color channel has 8 Bits.
         /// </summary>
-        Bit24,
+        Bit24 = 24,
     }
 }