Blender Git Loki

Fix T53459, inconsistent bevel on identical edges.

The old algorithm depended on vertex order.
The new one uses a global least squares solution on chains
and cycles of edges where loop slide induces a dependency.

See https://wiki.blender.org/index.php/Dev:Source/Modeling/Bevel
in the "Consistent Widths for Even Bevels" for derivation of
the new algorithm.