Blender Git Loki

Four mirrored Weight Paint improvement/fixes:

- If the mirrored group doesn't exist yet, it creates that group now.
- Painting on any non-mirror group (not a name complying mirror rules), it
will now paint mirrored on the group itself.
- The Mesh octree for mirror painting was not created on loading a file
in WeightPaint mode, or on exiting Editmode for Mesh. This caused mirror
painting to not work always.
- Tweaked the threshold calculation for vertices, so it gives fewer misses.