GlfwApplication: add mainLoopIteration similar to Sdl2Application

src/Magnum/Platform/GlfwApplication.cpp

@@ -603,14 +603,19 @@ void GlfwApplication::setSwapInterval(const Int interval) {
 int GlfwApplication::exec() {
     CORRADE_ASSERT(_window, "Platform::GlfwApplication::exec(): no window opened", {});
 
-    while(!glfwWindowShouldClose(_window)) {
+    while(mainLoopIteration()) {}
+
+    return _exitCode;
+}
+
+bool GlfwApplication::mainLoopIteration() {
     if(_flags & Flag::Redraw) {
         _flags &= ~Flag::Redraw;
         drawEvent();
     }
     glfwPollEvents();
-    }
+
-    return _exitCode;
+    return !glfwWindowShouldClose(_window);
 }
 
 namespace {

src/Magnum/Platform/GlfwApplication.h

@@ -238,6 +238,18 @@ class GlfwApplication {
          */
         int exec();
 
+        /**
+         * @brief Run one iteration of application main loop
+         * @return @cpp false @ce if @ref exit() was called and the application
+         *      should exit, @cpp true @ce otherwise
+         *
+         * Called internally from @ref exec(). If you want to have better
+         * control over how the main loop behaves, you can call this function
+         * yourself from your own `main()` function instead of it being called
+         * automatically from @ref exec().
+         */
+        bool mainLoopIteration();
+
         /**
          * @brief Exit application main loop
          * @param exitCode  The exit code the application should return