In this commit I changed many loops to range-based for loops. I also removed some of the redundant iterator variables, using indexing inside the loop instead. Generally an optimizing compiler should have no problem doing the smartest thing in that situation, and this way makes it much easier to tell where data is coming from. I only changed the loops I was confident about, so there is still more that could be done in the future. Differential Revision: https://developer.blender.org/D13637