This is a typical blender asset usage approach.  Linked
content can be included in a new blend file but it cannot be edited in
the file that links it.

Upgraded the blender exporter to understand linked groups and add them
to an exported scene.

Added "webgl_loader_scenegroup_blender.html".  This example shows
blender linked groups to demonstrate changes made to the
blender exporter.  The table.blend links an object group (cup and
silverware) from the dinnerset.blend file.  This example does not
incorporate the location offsets in the group definition which it really
should.  Being a new threejs user I did not know how best to do it.

This is now faster - losses by sorting are smaller than gains by rendering less pixels.

I don't know if the splitting is really needed. Will add back if it is.

allows a reduction to a face3 in Geometry's .mergeVertices(). so finally the
LatheGeometry example in subdivision surfaces is finally working as
intended!

refactor ParametricGeometry for new API. discarding other changes to rebase on mrdoob new refactoring