メッシュの頂点属性は MeshRenderer のプライマリーメッシュの属性に上書き追加します。
This can be used for vertex painting tools, etc. This data is serialized.
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.