For spherical spot light, when the shading point is close to the light
source, we switch to sampling the light spread instead of the visible
cone from the shading point. This has the benefit of less noise when the
spread is small.
However, the light spread sampling was not considering non-uniform
object scaling, where the actual spread might be different.
This patch switches sampling method only when the smallest enclosing
spread cone is smaller than the visible cone from the shading point.
An alternative method would be to compute the actual solid angle of the
scaled cone, and sample from the scaled cone. However, that involves
ray transformation and modifying the sampling pdf and angle. Since
non-uniform scaling is rather a niche case, it's probably not worth the
computation effort.
Weizhen Huang