Fixed a few places where lab color space still referred to the channels

as being signed rather than unsigned.

dlib/matrix/matrix_utilities.h

@@ -2750,9 +2750,9 @@ namespace dlib
                 const M& m
         )
         {
-            pixel.l = static_cast<signed char>(m(0));
-            pixel.a = static_cast<signed char>(m(1));
-            pixel.b = static_cast<signed char>(m(2));
+            pixel.l = static_cast<unsigned char>(m(0));
+            pixel.a = static_cast<unsigned char>(m(1));
+            pixel.b = static_cast<unsigned char>(m(2));
         }
     };
 

dlib/pixel.h

@@ -84,14 +84,14 @@ namespace dlib
                 - is_unsigned == true
              - else if (lab == true) then
                 - The type T will be a struct with 3 public members of type
-                  signed char named "l" "a" and "b".
+                  unsigned char named "l" "a" and "b".
                 - This type of pixel represents the Lab color space.
                 - num == 3
                 - has_alpha == false
                 - basic_pixel_type == unsigned char
                 - min() == 0
                 - max() == 255
-                - is_unsigned == false
+                - is_unsigned == true 
             - else
                 - grayscale == true
                 - This type of pixel represents a grayscale color space.  T