Update GLFW to 3.3.1

src/external/glfw/src/null_monitor.c

@@ -2,7 +2,7 @@
 // GLFW 3.3 - www.glfw.org
 //------------------------------------------------------------------------
 // Copyright (c) 2016 Google Inc.
-// Copyright (c) 2006-2016 Camilla Löwy <elmindreda@glfw.org>
+// Copyright (c) 2016-2019 Camilla Löwy <elmindreda@glfw.org>
 //
 // This software is provided 'as-is', without any express or implied
 // warranty. In no event will the authors be held liable for any damages
@@ -49,6 +49,12 @@ void _glfwPlatformGetMonitorContentScale(_GLFWmonitor* monitor,
         *yscale = 1.f;
 }
 
+void _glfwPlatformGetMonitorWorkarea(_GLFWmonitor* monitor,
+                                     int* xpos, int* ypos,
+                                     int* width, int* height)
+{
+}
+
 GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* found)
 {
     return NULL;