fix critical issue w/ ofVbo shallow copies (custom attributes)

* fixes an issue where custom vbo attributes (and their buffers) would not be retained whenever the vbo was shallow copied, since the copy constructors would not copy them over from their mom object.
* this is might have especially tripped you up when you wanted to create & then store vbos in a c++ vector, since push_back does initialise by copy, and you'd have lost all your attributes upon pushing the new ofVbo to your storage vector...

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2843 from tgfrerer/fix-critical-vbo-attribute-copy

fix critical issue w/ ofVbo shallow copies (custom attributes)