Define AbstractShaderProgram::Attribute<T>::Type as T.

The original comment is now saying the truth.

src/Magnum/AbstractShaderProgram.h

@@ -959,6 +959,14 @@ template<UnsignedInt location, class T> class AbstractShaderProgram::Attribute {
             VectorCount = Implementation::Attribute<T>::VectorCount
         };
 
+        /**
+         * @brief Type
+         *
+         * Type used in shader code.
+         * @see @ref ScalarType, @ref DataType
+         */
+        typedef T Type;
+
         /**
          * @brief Scalar type
          *