Use a bounding sphere instead of the corners of a bounding box to
compute the subtended angle of a light tree node.
Using the corners of the bounding box was an underestimate in some
scenes, causing some light tree nodes being incorrectly skipped.
Using the subtended angle of a bounding sphere is an overestimate, but
it covers the entire node and would not skip any valid contribution,
and no other reliable algorithm to compute the minimal enclosing angle
is known to us.
We expect some increase in noise due to overestimation, but this has
not been observed yet, in our benchmark scenes only a difference in
noise is visible.
Thanks to Weizhen for the suggestion to use the bounding sphere.
Pull Request: https://projects.blender.org/blender/blender/pulls/126625
84bab7f300