Fixes for compiling under OSX 10.8 with clang

Hi,

Some private fields in OpenNI2 classes are colliding with Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn) that uses -Werror,-Wunused-private-field. So as to make OpenNI2 compile, it did 2 kinds of hacks:
- promoting from private to protected
- commenting out the unused private class members.

I don't know about the OpenNI2 architecture, hence the hacks. Could you please review these?

Best regards,
Christian