Exposed new properties from RenderPackage

This pull request exposes two new properties on ```RenderPackage``` class:

```cs
PointVertexColors
TriangleVertexColors
```

These are not currently used anywhere (they simply return the internal data members just like all other properties before them), but as soon as the interface ```IRenderPackage``` has them, it will be extremely helpful. More on that next time.

Hi @pboyer, could you help me to take a quick look at these? Thanks a ton!