When `delta` is significantly larger than the difference between `tmin`
and `tmax`, the precision of `theta_a` and `theta_b` reduces, resulting
in banding artefacts.
For equiangular sampling, we use uniform sampling to fix this case.
For light tree, we use the equality
atan(a) + atan(b) = atan2(a + b, 1 - a*b).
Pull Request: https://projects.blender.org/blender/blender/pulls/142845
bb689687a7