Controls the sampling method for the world material. Selecting Auto or Manual enables
Multiple Importance Sampling while None disables it. Multiple Importance Sampling
is a method to sample the background texture such that lighter parts are favored,
creating an importance map. It will produce less noise in the render in trade of artifacts (Fireflies).
Enable this when using an image texture with small area lights (like the sun),
otherwise noise can take a long time to converge.
下面是启用和禁用多重重要性示例之间的比较。两种图像均渲染25秒（关闭：1,500个样本，打开：1,000个样本）。
多重重要性采样关闭。
多重重要性采样开启。