This function is used to add a system to a scene.
void dg_scene_add_sys(dg_scene_t *scene, dg_system_t *system);
dg_scene_add_sys(scene, &dg_sys_render);
void dg_scene_add_sys(dg_scene_t *scene, dg_system_t *system)
{
if (!scene || !system)
return;
dg_arr_add_begin(&(scene->systems), system);
}