We can't simply dupalloc cache_system (LaplacianSystem), it has quite a few allocated data we'd need to dupalloc as well,
not to mention (nl) context... Much safer to just set it to NULL in new copy imho!
This commit is to be backported to 2.70 release!
The laplacian modifiers (smooth and deform) use the OpenNL library,
which is not threadsafe due to the use of a global context variable.
Ideally this would be changed so that an explicit context can be
created for every caller of the OpenNL functions, but since OpenNL's
most recent version is from 2010 this is unlikely to happen.
As a workaround for now just use a mutex to prevent conflicting OpenNL
calls. Eventually OpenNL can be replaced by eigen or ceres.
