Alembic: Allow exporting of animated vertex colors
Allow exporting of animated vertex colors to Alembic.
The changes are made to be in line with the way the UV Maps are written. Each vertex color gets a OC4fGeomParam created and mapped into the CDStreamConfig to avoid recreating the Param on each frame.
The time sample index is also stored in the config now and set onto the UV and Vertex Color params each frame. Without this the exports would get inconsistent timing results where animated UV maps and Vertex Colors were not playing back at the original speed.