Follow up to 5d40992fe8.
In the PBVH API, some contexts that take a `float *` parameter named
`depth` use it purely as a return value, other places, however, read
from and write to this parameter.
In the former case, this commit either adds or keeps the `r_` prefix, in
the latter, this commit ensures the parameter has no prefix to avoid
misleading future readers.
Pull Request: https://projects.blender.org/blender/blender/pulls/127886
e37f5616f5