Avoid mode re-enumeration.

src/monitor.c

@@ -340,8 +340,11 @@ GLFWAPI const GLFWvidmode* glfwGetVideoModes(GLFWmonitor* handle, int* count)
 
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
+    if (monitor->modes == NULL)
+    {
         if (!refreshVideoModes(monitor))
             return GL_FALSE;
+    }
 
     *count = monitor->modeCount;
     return monitor->modes;