When the spread of a spot light is at it's maximum (180 degrees),
then `atan(tan(theta_e))` could become quite unpredictable due to
a asymtote in the tan function when working with lights with this
spread (because theta_e is `spread * 0.5 = pi/2`).
This lead to issues like theta_e for the spotlight becoming negative,
which lead to rendering errors due to a malformed light tree.
This commit fixes this issue by adding a episilon region around the
troublesome values and sets theta_e to `pi/2` when in that region.
Candidate for backporting to 4.2 and potentially 3.6
Pull Request: https://projects.blender.org/blender/blender/pulls/125172
90e83175eb