A set of general tidying steps prior to making some PBVHVertRef changes
* Add const where possible
* Add r_ prefix for return parameters
* Use pass-by-reference instead of pointer for some return parameters
* Use std::array's .size() operator for some iteration bounds
* Use float3 for some variables
Pull Request: https://projects.blender.org/blender/blender/pulls/127832
5d40992fe8