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/threads.c

@@ -124,7 +124,7 @@ int main(void)
 
         for (i = 0;  i < count;  i++)
         {
-            if (glfwGetWindowParam(threads[i].window, GLFW_SHOULD_CLOSE))
+            if (glfwWindowShouldClose(threads[i].window))
                 running = GL_FALSE;
         }
     }