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 ofVbo was copied, since the ofVbo copy constructors would not include custom attribute data & GL handlers when assigning data from their mom object.

## note 

this is might have especially tripped you up if you wanted to create, and then store vbos with custom attributes 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>