Blender Git Loki

Fix for name collisions in compound properties when writing CustomData.

This is not clearly documented for Alembic, but apparently properties
inside compounds still need to be name uniquely for the whole object.
This is somewhat involved for Blender's CustomData, because we have
5 different customdata elements for meshes (vert, edge, face, poly,
loop) and each of these can have the same types and multiple layers of
the same type ...