Math: directly access Vector components in gather() / scatter().

Speeding up debug builds a bit.

src/Magnum/Math/Swizzle.h

@@ -38,7 +38,7 @@ namespace Implementation {
         static_assert(size > position, "swizzle parameter out of range of gather vector");
 
         template<class T> constexpr static T value(const Math::Vector<size, T>& vector) {
-            return vector[position];
+            return vector._data[position];
         }
     };
 
@@ -70,7 +70,7 @@ namespace Implementation {
             "swizzle parameter out of range of scatter vector");
 
         template<class T> constexpr static T value(const Math::Vector<size, T>& vector, const T&) {
-            return vector[i];
+            return vector._data[i];
         }
     };
     template<std::size_t size, std::size_t position> struct Value {

src/Magnum/Math/Vector.h

@@ -81,6 +81,9 @@ namespace Implementation {
 
     /* Used to make friends to speed up debug builds */
     template<std::size_t, class> struct MatrixDeterminant;
+    /* To make gather() / scatter() faster */
+    template<std::size_t, std::size_t> struct ComponentAtPosition;
+    template<std::size_t, char, std::size_t> struct ComponentOr;
 }
 
 /** @relatesalso Vector
@@ -652,6 +655,9 @@ template<std::size_t size, class T> class Vector {
         template<std::size_t, std::size_t, class> friend class RectangularMatrix;
         template<std::size_t, class> friend class Matrix;
         template<std::size_t, class> friend struct Implementation::MatrixDeterminant;
+        /* To make gather() / scatter() faster */
+        template<std::size_t, std::size_t> friend struct Implementation::ComponentAtPosition;
+        template<std::size_t, char, std::size_t> friend struct Implementation::ComponentOr;
 
         /* So the out-of-class comparators can access data directly to avoid
            function call overhead */