Add tracking of which window 'owns' a monitor

src/internal.h

@@ -381,6 +381,9 @@ struct _GLFWmonitor
     // Physical dimensions in millimeters.
     int             widthMM, heightMM;
 
+    // The window whose video mode is current on this monitor
+    _GLFWwindow*    window;
+
     GLFWvidmode*    modes;
     int             modeCount;
     GLFWvidmode     currentMode;
@@ -911,6 +914,10 @@ void _glfwInputCursorEnter(_GLFWwindow* window, GLFWbool entered);
  */
 void _glfwInputMonitorChange(void);
 
+/*! @ingroup event
+ */
+void _glfwInputMonitorWindowChange(_GLFWmonitor* monitor, _GLFWwindow* window);
+
 /*! @brief Notifies shared code of an error.
  *  @param[in] error The error code most suitable for the error.
  *  @param[in] format The `printf` style format string of the error