Platform: avoid using 100% CPU in an idle GlfwApplication.

doc/changelog.dox

@@ -505,6 +505,8 @@ See also:
     class construction, potentially causing crashes or
     `pure virtual method call` aborts. To prevent these issues, event callback
     setup is delayed to the first time the application main loop is entered.
+-   @ref Platform::GlfwApplication was polling for events instead of waiting,
+    using 100% CPU even if not constantly redrawing
 -   In 2019.01 @ref Magnum/Platform/Sdl2Application.h went through an include
     cleanup, removing 50k lines; but unfortunately we forgot to add back
     @cpp #include <SDL_main.h> @ce, causing iOS builds to fail to initialize.

src/Magnum/Platform/GlfwApplication.cpp

@@ -741,11 +741,14 @@ bool GlfwApplication::mainLoopIteration() {
     */
     if(glfwGetWindowUserPointer(_window) != this) setupCallbacks();
 
+    /* If redrawing, poll for events immediately after drawEvent() (which could
+       be setting the Redraw flag again, thus doing constant redraw). If not,
+       avoid spinning the CPU by waiting for the next input event. */
     if(_flags & Flag::Redraw) {
         _flags &= ~Flag::Redraw;
         drawEvent();
-    }
-    glfwPollEvents();
+        glfwPollEvents();
+    } else glfwWaitEvents();
 
     return !glfwWindowShouldClose(_window);
 }