Add extra logging to debug the event loop

This should make tracking down the root cause of the
event loop pauses on X11 easier. And the infrastructure
should come in handy in the future as well.

glfw/main_loop.h

@@ -29,6 +29,7 @@ void _glfwPlatformRunMainLoop(GLFWtickcallback callback, void* data) {
     keep_going = GLFW_TRUE;
     tick_callback_requested = GLFW_FALSE;
     while(keep_going) {
+		EVDBG("tick_callback_requested: %d", tick_callback_requested);
         while (tick_callback_requested) {
             tick_callback_requested = GLFW_FALSE;
             callback(data);