Added AbstractShaderProgram::attachShaders().

Convenience overload to attachShader(), allowing the user to specify
more than one shader at once. Just a complement to initializer-list
versions of compile() and link(), but without any performance
difference.

src/Magnum/AbstractShaderProgram.cpp

@@ -262,6 +262,10 @@ void AbstractShaderProgram::attachShader(Shader& shader) {
     glAttachShader(_id, shader.id());
 }
 
+void AbstractShaderProgram::attachShaders(std::initializer_list<std::reference_wrapper<Shader>> shaders) {
+    for(Shader& s: shaders) attachShader(s);
+}
+
 void AbstractShaderProgram::bindAttributeLocation(UnsignedInt location, const std::string& name) {
     glBindAttribLocation(_id, location, name.data());
 }

src/Magnum/AbstractShaderProgram.h

@@ -614,6 +614,15 @@ class MAGNUM_EXPORT AbstractShaderProgram: public AbstractObject {
          */
         void attachShader(Shader& shader);
 
+        /**
+         * @brief Attach shaders
+         *
+         * Convenience overload to the above, allowing the user to specify more
+         * than one shader at once. Other than that there is no other
+         * (performance) difference when using this function.
+         */
+        void attachShaders(std::initializer_list<std::reference_wrapper<Shader>> shaders);
+
         /**
          * @brief Bind attribute to given location
          * @param location      Location