New packages: wip/prusaslicer & deps

[Paul Ripke <stix%stix.id.au@localhost>]

wip/prusaslicer, and deps:
wip/cereal
wip/openvdb
wip/nlopt (upgrade of math/nlopt)

Seems reliable in my testing, but I've only tested on NetBSD 9.1 amd64.
pkglint and verifypc are as clean as I can get them (prusaslicer throws
warnings for a whole bunch of uninteresting unresolvable deps).

Note that graphics/blender can also build against openvdb, rather than
using a built-in copy, and also seems to work fine.

I don't have any other readily available build environments, except
NetBSD 9.1 on armv7 & MacOS X amd64, and I don't think either of those
is that useful.

Also note that there's a dependency on
parallel/threadingbuildingblocks which currently only supports
x86 32/64, for silly reasons. I've got a modified package working on
armv7, and should also work elsewhere. I'll kick off a build of
wip/prusaslicer there and see how it goes.

-- 
Paul Ripke
"Great minds discuss ideas, average minds discuss events, small minds
 discuss people."
-- Disputed: Often attributed to Eleanor Roosevelt. 1948.