延迟光照（旧版）。
This is a deferred lighting path (also known as "light pre-pass"), that
renders Scene information into a very small G-Buffer twice, computing lighting in between.
它不需要 GPU 具有多渲染目标 (MRT) 支持，但
灵活性比 RenderingPath.DeferredShading 要低。
注意：延迟渲染与正交摄像机投影不兼容。