fix: is_equal_to_sixteen in PNG I/O was less-than test (#650)

While is_equal_to_sixteen should (as the name suggests) check for
equality to 16, it actually checked whether the bit depth was
less than 16.

Author: striezel

include/boost/gil/extension/io/png/detail/write.hpp

@@ -171,10 +171,24 @@ class writer< Device
     {};
 
     template<typename Info>
-    struct is_equal_to_sixteen : mp11::mp_less
+    struct is_equal_to_sixteen : mp11::mp_and
         <
-            std::integral_constant<int, Info::_bit_depth>,
-            std::integral_constant<int, 16>
+            mp11::mp_not
+            <
+                mp11::mp_less
+                <
+                    std::integral_constant<int, Info::_bit_depth>,
+                    std::integral_constant<int, 16>
+                >
+            >,
+            mp11::mp_not
+            <
+                mp11::mp_less
+                <
+                    std::integral_constant<int, 16>,
+                    std::integral_constant<int, Info::_bit_depth>
+                >
+            >
         >
     {};