Vertex attributes in this mesh will override or add attributes of the primary mesh in the MeshRenderer.
This is used for the UVs for realtime lightmaps, but could also be used for vertex painting tools, etc.
For example, if the primary mesh doesn't contain the Mesh.colors channel, the colors from additionalVertexStreams are used. If the primary mesh contains the colors channel and the additionalVertexStreams mesh also has colors, then colors from additionalVertexStreams are used.
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