Changed the pixel_traits so that signed grayscale pixel types are allowed. This

involved adding a few new fields to pixel_traits.  I also changed the get_pixel_intensity()
function so that its return value is of the same type as the basic pixel type rather
than always being unsigned long.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403967

dlib/gui_core/gui_core_kernel_1.h

@@ -129,7 +129,10 @@ namespace dlib
         const static bool grayscale = false;
         const static bool hsi = false;
         const static long num = 3;
-        static unsigned long max() { return 255;}
+        typedef unsigned char basic_pixel_type;
+        static basic_pixel_type min() { return 0;}
+        static basic_pixel_type max() { return 255;}
+        const static bool is_unsigned = true;
         const static bool has_alpha = false;
     };
 

dlib/gui_core/gui_core_kernel_2.h

@@ -145,7 +145,10 @@ namespace dlib
         const static bool grayscale = false;
         const static bool hsi = false;
         const static long num = 3;
-        static unsigned long max() { return 255;}
+        typedef unsigned char basic_pixel_type;
+        static basic_pixel_type min() { return 0;}
+        static basic_pixel_type max() { return 255;}
+        const static bool is_unsigned = true;
         const static bool has_alpha = false;
     };