Replaced window close parameter with mutable flag.

Replaced the GLFW_SHOULD_CLOSE window parameter with the
glfwWindowShouldClose and glfwSetWindowShouldClose functions, allowing
the setting of the close flag from any point in the program.

tests/events.c

@@ -416,7 +416,7 @@ int main(void)
 
     printf("Main loop starting\n");
 
-    while (!glfwGetWindowParam(window, GLFW_SHOULD_CLOSE))
+    while (!glfwWindowShouldClose(window))
         glfwWaitEvents();
 
     glfwTerminate();