Blender Git Loki

Edit Mesh: Use multithreading in other parts of Auto Merge & Split

In the Auto Merge & Split feature, multithreading was only used to
find duplicates between vertex and another vertex.

But with this patch, multithreading is now used to find intersections
etween edge and edge and between edge and vertex.

In my tests I noticed a performance improvement of around 180%
(0.017151 secs to 0.009373 secs)

Differential Revision: https://developer.blender.org/D6528