Part of T70240
This is the initial implementation of Weld Modifier.
New features will be added later.
ToDo:
- Seams: restrict welding to vertices along boundary edges.
- Edge Collapse: collapse edges below the length threshold.
- New icon.
- Some customdata are not being correctly interpolated.
Differential Revision: https://developer.blender.org/D6383
f050fa3253