Optimization of the Cycles renderer for layered rendering（ cycles渲染器对于分层渲染的优化）

In the compositing workflow, we typically disassemble and render the entire beauty layer before performing the compositing in a compositor (compositing software). Sometimes, we may only require certain individual channels, such as a fog field channel. When we render only a single fog field channel, Cycles needs to render all elements, which is rather time - consuming. I propose that when we only need a single channel, Cycles should perform only the necessary rendering. This approach will significantly reduce the rendering time and offer greater flexibility.（在合成的工作流中，我们通常会把beauty层全部拆解出来渲染，再到合成器（合成软件中进行合成），有时候我们可能只需要一些单独的通道，比如说一个雾场通道，当我们只渲染一个雾场通道时，cycles会需要把所有的都渲染一遍，这将比较耗费时间。我认为可以修改为当我们只需要单个通道时，cycles只进行必要的渲染，这将减少很多的渲染时间，也更加的灵活）