Transform Snap: Optimize edge-snap using BVH tree

changes in BLI_kdopbvh:

- `BLI_bvhtree_find_nearest_to_ray` now takes is_ray_normalized and scale argument.
- `BLI_bvhtree_find_nearest_to_ray_angle` has been added (use for perspective view).

changes in BLI_bvhutils:

- `bvhtree_from_editmesh_edges_ex` was added.

changes in math_geom:

- `dist_squared_ray_to_seg_v3` was added.

other changes:

- `do_ray_start_correction` is no longer necessary to snap to verts.
- the way in which the test of depth was done before is being simulated in callbacks.

source/blender/blenlib/BLI_kdopbvh.h

@@ -96,7 +96,8 @@ typedef void (*BVHTree_NearestPointCallback)(void *userdata, int index, const fl
 typedef void (*BVHTree_RayCastCallback)(void *userdata, int index, const BVHTreeRay *ray, BVHTreeRayHit *hit);
 
 /* callback must update nearest in case it finds a nearest result */
-typedef void (*BVHTree_NearestToRayCallback)(void *userdata, int index, const BVHTreeRay *ray, BVHTreeNearest *nearest);
+typedef void (*BVHTree_NearestToRayCallback)(void *userdata, const float ray_co[3], const float ray_dir[3],
+                                             const float scale[3], int index, BVHTreeNearest *nearest);
 
 /* callback to check if 2 nodes overlap (use thread if intersection results need to be stored) */
 typedef bool (*BVHTree_OverlapCallback)(void *userdata, int index_a, int index_b, int thread);
@@ -142,8 +143,16 @@ int BLI_bvhtree_find_nearest(
         BVHTree *tree, const float co[3], BVHTreeNearest *nearest,
         BVHTree_NearestPointCallback callback, void *userdata);
 
+int BLI_bvhtree_find_nearest_to_ray_angle(
+        BVHTree *tree, const float co[3], const float dir[3],
+        const bool ray_is_normalized, const float scale[3],
+        BVHTreeNearest *nearest,
+        BVHTree_NearestToRayCallback callback, void *userdata);
+
 int BLI_bvhtree_find_nearest_to_ray(
-        BVHTree *tree, const float co[3], const float dir[3], BVHTreeNearest *nearest,
+        BVHTree *tree, const float co[3], const float dir[3],
+        const bool ray_is_normalized, const float scale[3],
+        BVHTreeNearest *nearest,
         BVHTree_NearestToRayCallback callback, void *userdata);
 
 int BLI_bvhtree_ray_cast_ex(