AbstractXApplication: add mainLoopIteration similar to Sdl2Application

src/Magnum/Platform/AbstractXApplication.cpp

@@ -126,7 +126,12 @@ int AbstractXApplication::exec() {
     /* Show window */
     XMapWindow(_display, _window);
 
-    while(!(_flags & Flag::Exit)) {
+    while(mainLoopIteration()) {}
+
+    return 0;
+}
+
+bool AbstractXApplication::mainLoopIteration() {
     XEvent event;
 
     /* Closed window */
@@ -172,9 +177,8 @@ int AbstractXApplication::exec() {
         _flags &= ~Flag::Redraw;
         drawEvent();
     } else Utility::System::sleep(5);
-    }
 
-    return 0;
+    return !(_flags & Flag::Exit);
 }
 
 void AbstractXApplication::viewportEvent(ViewportEvent& event) {

src/Magnum/Platform/AbstractXApplication.h

@@ -109,6 +109,18 @@ class AbstractXApplication {
          */
         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 */
         void exit() { _flags |= Flag::Exit; }