Folks, just to let you know: Don't get too accustomed to the exact
syntax; I have strong reason to suspect I'll change it.

For starters, there may be cases when you want to import an .obj not as
a single mesh but as a set of meshes. (Point in case: Poser or DAZ
figures with eyelashes; the skin should have a refractive index of
around 1.45 for the Fresnelian effects to be modeled properly, while the
eyelashes must not have a refractive index at all or they'll look weird;
and while we're at it, the teeth should have a reflective index of
around 1.6. But POV-Ray does not support different refractive indices in
a single mesh.) The .obj mesh import syntax could be changed to import
only geometry that matches certain criteria (based on material, group or
object), but this would mean that in order to import all parts of an
.obj file it would have to be parsed multiple times, which isn't efficient.

So the approach I'll probably take is to make the obj import feature
create a single mesh if no filtering is done, but also provide means to
create a CSG union of meshes. But obviously it would be a tad confusing
to have a `mesh` statement create a CSG.

Also, in case we ever extend the import to support non-mesh geometry
(obj also supports points, lines and various NURBS-ish stuff) or even
other file formats that may support genuine geometric primitives,
stuffing all of it into the `mesh` syntax would seem rather odd.

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