The code for recalculating bounds was fairly tricky to understand, with a
mixture of various flags, a mixture of recursive and parallel code, and
the "original bounds" concept.
This commit separates that update code into two separate paths. The regular
bounds update is split from the resetting of original bounds, which is only
needed in a few places and is best described by a separate function call.
The bounds update itself is split clearly into a parallel update of leaf node
bounds and a recursive merging of child node bounds.
The high level goals this helps to support are:
1. Use data-oriented storage to replace PBVHNode struct.
2. Document concepts like "original bounds", provide a self-descriptive API.
3. Allow brushes to update bounds themselves, for better CPU cache use.
Pull Request: https://projects.blender.org/blender/blender/pulls/122746
f9a217a276