Vertex channel conversion

In the XNA content pipeline vertex colors are usually imported as `VertexChannel<Vector4>`.
In the MonoGame content pipeline vertex colors seem to be imported as `VertexChannel<Color>`.
That's just a minor difference - no problem so far.

When the mesh is processed in my custom model processor, I call 

    geometryContent.Vertices.Channels.ConvertChannelContent<Color>(colorChannelIndex);

This call fails in MonoGame because `VertexChannel<Color>` cannot be converted to `VertexChannel<Color>`. This commit adds support for this trivial case.