Add some color literals.

    * mln/colors/literal.hpp: New literals.

mln/colors/literal.hpp

@@ -19,9 +19,14 @@ namespace mln
       //using blue_t = literal_color<rgb8, rgb8{0,0,255}>;
       namespace
       {
+        constexpr rgb8 black{0,0,0};
         constexpr rgb8 red{255,0,0};
         constexpr rgb8 green{0,255,0};
         constexpr rgb8 blue{0,0,255};
+        constexpr rgb8 white{255,255,255};
+        constexpr rgb8 cyan{0,255,255};
+        constexpr rgb8 yellow{255,255,0};
+        constexpr rgb8 magenta{255,0,255};
       }
 
     }