Made tangent frame consistent across the surface regardless of the sample,
which was not the case with the previous algorithm. Previously, a tangent
frame would stay consistent for the same sample throughout the walk, but not
from sample to sample for the same triangle. This actually resulted in code
simplification.
Also includes additional fixes:
* Fixed an important bug that manifested itself with multiple lights in the
scene, where caustics had abnormally low amplitude: The final light pdf did
not include the light distribution pdf.
* Removed unnecessary orthonormal basis generation function, using cycles'
native one instead.
* Increased solver max iteration back to 64: It turns out we sometimes need
these extra iterations in cases where projection back to the surface takes
many steps. The effective solver iteration count, the most expensive part,
is actually much less than the raw iteration count.
Differential Revision: https://developer.blender.org/D14931
48754bc146