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These runtime warnings are not helpful, it may be intentional to have
an object that sometimes does not contain entities of a specific kind,
without changing the drawmode when this happens.

```
warning: numIndices is 0 in renderobject: VolumeMeshNodeVertices
warning: numIndices is 0 in renderobject: VolumeMeshNodeFaces
warning: numIndices is 0 in renderobject: VolumeMeshNodeEdges
```

implements picking for core profiles

renderer is used at startup. fixes #1

implementation for core profiles.
* Implemented first variant for picking volumeḾeshvertices in core
profiles.