Blender Git Loki

Fix T41191: Face snapping doesn't work properly.

Issue was caused by rB47ec0394ca3d, which disabled BBox check in editmode - but bbox check was also
setting `len_diff`, which is mandatory when doing ray_start_local correction for ortho view...

Now, in this case, we do a quick rough compute of len_diff from vertices coordinates (accuracy is not
needed here, we just have to be sure corrected `ray_start_local` remains 'before' (outside) of the
geometry).