Update STLSlice to improve calculation of lower shell.
Update STLSlice to improve calculation of lower shell.

git-svn-id: d2924805b4ae8fffc34e1a97dde5dc7532b2300d@453 cb376a5e-1013-0410-a455-b6b1f9ac8223

alx/reprap-host-software,alx/reprap-host-software