Only set the monitor callback once.

tests/events.c

@@ -476,6 +476,8 @@ int main(int argc, char** argv)
 
     printf("Library initialized\n");
 
+    glfwSetMonitorCallback(monitor_callback);
+
     while ((ch = getopt(argc, argv, "hfn:")) != -1)
     {
         switch (ch)
@@ -557,8 +559,6 @@ int main(int argc, char** argv)
 
         glfwSetWindowUserPointer(slots[i].window, slots + i);
 
-        glfwSetMonitorCallback(monitor_callback);
-
         glfwSetWindowPosCallback(slots[i].window, window_pos_callback);
         glfwSetWindowSizeCallback(slots[i].window, window_size_callback);
         glfwSetFramebufferSizeCallback(slots[i].window, framebuffer_size_callback);