Platform: use Flag::Redraw instead of a boolean in GlfwApplication.

Also properly reset it before drawEvent() so the users can control the
amount of redrawing themselves.

src/Magnum/Platform/GlfwApplication.cpp

@@ -52,7 +52,7 @@ GlfwApplication::GlfwApplication(const Arguments& arguments, const Configuration
 
 GlfwApplication::GlfwApplication(const Arguments& arguments, std::nullptr_t):
     _context{new Context{NoCreate, arguments.argc, arguments.argv}},
-    _needsRedraw(true)
+    _flags{Flag::Redraw}
 {
     /* Save global instance */
     _instance = this;
@@ -154,7 +154,8 @@ void GlfwApplication::setSwapInterval(const Int interval) {
 
 int GlfwApplication::exec() {
     while(!glfwWindowShouldClose(_window)) {
-        if(_needsRedraw) {
+        if(_flags & Flag::Redraw) {
+            _flags &= ~Flag::Redraw;
             drawEvent();
         }
         glfwPollEvents();

src/Magnum/Platform/GlfwApplication.h

@@ -183,7 +183,7 @@ class GlfwApplication {
         void setSwapInterval(Int interval);
 
         /** @copydoc Sdl2Application::redraw() */
-        void redraw() { _needsRedraw = true; }
+        void redraw() { _flags |= Flag::Redraw; }
 
     #ifdef DOXYGEN_GENERATING_OUTPUT
     protected:
@@ -241,6 +241,13 @@ class GlfwApplication {
         /*@}*/
 
     private:
+        enum class Flag: UnsignedByte {
+            Redraw = 1 << 0
+        };
+
+        typedef Containers::EnumSet<Flag> Flags;
+        CORRADE_ENUMSET_FRIEND_OPERATORS(Flags)
+
         static void staticViewportEvent(GLFWwindow*, int w, int h) {
             _instance->viewportEvent({w, h});
         }
@@ -259,9 +266,11 @@ class GlfwApplication {
 
         GLFWwindow* _window;
         std::unique_ptr<Platform::Context> _context;
-        bool _needsRedraw;
+        Flags _flags;
 };
 
+CORRADE_ENUMSET_OPERATORS(GlfwApplication::Flags)
+
 /**
 @brief Configuration