Require at least one slice when initializing sliced geometry on objects with non-zero extents

Code expects in multiple places that proper SliceGeometry3Ds
(at least evenly-spaced ones) have plane geometries, i.e. at least
one slice.
This change remedies violated assertions when loading very small
triangle meshes (e.g. extent 0.3 in one dimension).

Signed-off-by: Daniel Maleike <code@maleike.de>