Blender Git Loki

Weld Modifier: Use KD Tree in detecting the geometry to be welded

This commit replaces the BVH Tree currently used by the Weld Modifier
with the KD Tree used by `Merge > By Distance`.

This changes the result of the Weld Modifier to more closely match
`Merge > By Distance`.

There is also a big performance advantage.

Here is an overview (models in D8995):

| 2.90 (Duplicate Limit = 0) | 2.90 (Duplicate Limit = 1) | master (BVH) (Duplicate Limit = 1) | patch (KD) |
| 1.69s| 0.17s | 0.12s | 0.029s |

Differential Revision: https://developer.blender.org/D8995