expose inner vbo in ofVboMesh

this ideally allows the wrapper to be a bit more flexible, especially by giving access to ofVbo::setAttributeData()

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2229 from tgfrerer/feature-ofVboMesh-to-expose-Vbo

expose inner vbo in ofVboMesh