Blender Git Loki

Fix for [#30438] x=zero not accepted for a bone tail.

In fact, problem was in vec_roll_to_mat3(), which has to detect when the bone is aligned with its Y axis, using a threshold. This one have been raised to quite a high value due to bug [#23954], then lowered a bit due to [#27675] (which is imho in fine the same problem as 30438).

Reset it to its org value (very low 1e-13), as testing file given with firt bug did not show any problem anymore... So now, instead of 1/1000 of bone length from Y axis, we have about 3.25*10-7... Only (hardly) noticeable at max zoom level in 3D view.