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Fix T62664: Exporting Metaballs as Alembic crashes Blender

The `AbcMetaballWriter` now subclasses `AbcGenericMeshWriter` instead of
wrapping an `AbcMeshWriter`. `AbcGenericMeshWriter` was created for this
purpose (writing non-mesh objects as mesh to Alembic) and performs the work
in a cleaner and, more importantly, not crashing way.