Shadow：先从光源渲染一张深度图

renderer.shadowMap.enabled = true;light.castShadow = true;light.shadow.bias = -0.0005;mesh.castShadow = true;ground.receiveShadow = true;light.position.x = -2.00;// shader 里大致是：// float lit = myDepth - bias < mapDepth ? 1.0 : 0.0;