This patch makes Cycles ignore the time spent in BVH construction etc. when
estimating the remaining time. Considering that the remaining time is calculated
based on the average time per tile so far, as far as I understand it makes no
sense to include the preprocessing time.
Reviewers: sergey, #cycles
Reviewed By: sergey, #cycles
Subscribers: sergey
Projects: #cycles
Differential Revision: https://developer.blender.org/D895