Blender Git Loki

Alembic export: separated "writing Alembic mesh" from "writing mesh object"

I moved most of the `AbcMeshWriter` code to a new class
`AbcGenericMeshWriter`. The latter is an abstract class and does not
make any assumptions about the type of Blender object being written.
This makes it possible to write metaballs, curves, nurbs surfaces, etc.
as mesh to Alembic files.

The `AbcMeshWriter` class now is the concrete implementation of
`AbcGenericMeshWriter` for writing mesh objects.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D4213