Fix T81467: Crash with KD-Tree Weld Modifier

The problem is related to the `use_index_order` option of `BLI_kdtree_3d_calc_duplicates_fast`.
With this option, the higher index is expected to be less than `tree->nodes_len`.

Fix T81060: CustomData Correction sometimes breaks UVs and Vertex Colors

`CustomData_bmesh_interp` use the same CustomData decryptor (in this case, `bm->ldata`) in both blocks.

So make sure that all CustomData layers match.

This commit also removes redundant `BM_elem_attrs_copy_ex` calls.

Maniphest Tasks: T81060

Differential Revision: https://developer.blender.org/D9159

Cleanup: Simplify use of 'Snapping Gears'

This commit removes `t->snap[0]` and `t->snap_spatial[0]`.

They were not actually being used, and could add overhead for
transformation without snap.

Cleanup: Simplify transform_orientation_matrix_get

This simplifies obtaining the parameters for internal functions.
(switch was complicating a little)

Also removes ureachable code.

Cleanup: Move transform_orientations functions to separate header

Improves the organization and identification of the API.

Fix T81529: Can't move with constraint to a zero-size local axis

To fix the problem, it was necessary to create a fallback for the
zero-sized axis in local orientation.

This also affects the gizmos.

Cleanup: Remove redundant if-condition

Fix T81480: Input 0 to scale in redo panel resets to 1

Checking if `t->values[i] != 0.0f;` is not the best solution for all
transform cases.

Fix T79973: Re-ordering face maps messes up the names of other face maps

Use a remap function instead a swap.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D8739

Fix T80520: Tris to Quads ignores UV delimit option