Fix: remove accidental code

`v1` and `v2` are already set.

Optimization: use `BLI_bvhtree_intersect_plane` to detect faces that will be affected by the knife tool

The knife code currently calls the `BLI_bvhtree_overlap` function that
tests the overlap between the mesh tree and an AABB that encompasses the
points projected in the clip_start, clip_end and or clip_planes of the
view.

This resulted in many false positives since the AABB is very large.
Often all the triangles "overlapped".

The solution was to create a new function that actually tests the
intersection of AABB with a plane.

Even not considering the clip_planes of the view, this solution is more
appropriate than using overlap.

Differential Revision: https://developer.blender.org/D8229

Cleanup: Use the BLI_edgehash API in the sewing simulation of cloths

Also remove the code in the ghash that is no longer used.

This change simplifies the existing code.

Differential Revision: https://developer.blender.org/D8219

Optimization: Don't compute the snap to face on the knife tool twice

Both `knife_find_closest_vert` and `knife_find_closest_edge` call
`knife_find_closest_face`. Thus, running the raycast twice and setting
values like `kcd->curr.bmface` and `kcd->curr.is_space` repeatedly.

So:
- separate `knife_find_closest_face` from `knife_find_closest_vert` and `knife_find_closest_edge`.
- rename `knife_find_closest_vert` to `knife_find_closest_vert_of_face`
- rename `knife_find_closest_edge `to `knife_find_closest_edge_of_face`.
- do not set parameters previously set.

Differential Revision: https://developer.blender.org/D8198

Fix faces disappearing when AutoMerge & Split

Fix T78570: Vert/Edge slide doesn't adjust UVs

Fix alignment test when snap to edge while in vert or edge slide

Revert "Fix alignment test when snap to edge while in constraint"

This reverts commit e16972389e728eeaf5043bb3cbd85fb7312a6463.

Fix alignment test when snap to edge while in constraint

Transform: generalized custom-data correction support

Support custom-data correction based on surrounding geometry for all
transformation modes of the mesh transform operators.

The is the same logic used in Vert and Edge Slide.

In order not to change the current default behavior,
this property does not affect Vert and Edge Slide modes.