Blender Git Loki

Fix for #1355

Recalculate normals (mesh editmode) had some weak points, as demonstrated
in the bug report sample file. It appeared to be 2 aspects that could be
simply fixed:

- for calculation of the most outer face (to set normal outside) it used
a manhattan distance. making it real distance helped quite some
- but most important: the routine only took triangles into account, which
didn't work proper with very irregular quads, especially with non-flat
quads. It now calculates both trias in a quad individually.