Blender Git Loki

Alembic export: added support for writing dupli-groups

This supports our common character animation workflow, where a character,
its rig, and the custom bone shapes are all part of a group. This group
is then linked into the scene, the rig is proxified and animated. Such
a group can now be exported. Use "Renderable objects only" to prevent
writing the custom bone shapes to the Alembic file.