Add ICC Lab interpretation

The CIE Lab interpretation uses heterogeneous samples, and the ITU Lab
encoding requires the Tag Decode (433, 0x01B1) which has default values
depending on bit depth and convering values into floating points, probably.

See also Photometric Interpretation: <https://web.archive.org/web/20050309053132/http://www.awaresystems.be/imaging/tiff/tifftags/photometricinterpretation.html>
See also Tag Decode: <https://web.archive.org/web/20050309072856/http://www.awaresystems.be/imaging/tiff/tifftags/decode.html>

src/lib.rs

@@ -44,6 +44,9 @@ pub enum ColorType {
     /// Pixel is YCbCr
     YCbCr(u8),
 
+    /// Pixel is CIE L*a*b* (ICC LAb)
+    Lab(u8),
+
     /// Pixel has multiple bands/channels
     Multiband { bit_depth: u8, num_samples: u16 },
 }
@@ -58,6 +61,7 @@ impl ColorType {
             | ColorType::CMYK(b)
             | ColorType::CMYKA(b)
             | ColorType::YCbCr(b)
+            | ColorType::Lab(b)
             | ColorType::Multiband { bit_depth: b, .. } => b,
         }
     }
@@ -72,6 +76,7 @@ impl ColorType {
             ColorType::CMYK(_) => 4,
             ColorType::CMYKA(_) => 5,
             ColorType::YCbCr(_) => 3,
+            ColorType::Lab(_) => 3,
             ColorType::Multiband { num_samples, .. } => num_samples,
         }
     }