modifyVertexStream(object, vertexBuffer, targetIndexing) -> self
This method gets called to allow the mutator to alter the data for a custom vertex stream.
Use the requirements in the vertexBuffer to get the description of the stream.
Use vertexBuffer.aquire() and vertexBuffer.commit() to fill the buffer.
* object (MObject) - The object being evaluated.
* vertexBuffer [IN/OUT] (MVertexBuffer) - The vertex buffer to alter.
* targetIndexing (MComponentDataIndexing) - Vertex index mapping from targetIndexing.getComponentType() space to vertex buffer space.