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Use BLI_bvhtree_walk_dfs for snapping

The snapping functions when performed in the perspective view,
have some problems in the threshold (a distortion) and in the clip plane (the normal is incorrect).
These problems can be only observed when making the snap to edges or to vertices (nearest to ray function).

This patch propose a totally different solution.
The idea is to project the edges of bvh nodes and test the 2d projection of the snap element.

For this it used the BLI_bvhtree_walk_dfs function.

It is important to pay particular attention also to the changes in `ED_transform_snap_object_project_view3d_ex`