This make sure only one line is drawn per edge.
It makes the function mesh_create_edit_loops_points_lines() non-thread safe
but this is fine as of now because nothing is multithreaded at this point.
Also this is the only function use this flag so it might be OK.
The side effect is that we don't need to use depth test in edit mode
overlay so the masking artifact will not appear.
ea30767997