OpenNI2 Driver Modification.

I noticed that you are main author of OpenNI2 driver, so I sent you this pull request .

My main modification point is below;
Microsoft Kinect cannot turn off mirror mode. setMirroringEnabled(false) always fails.
So we should flip the images when we copy 'openni::VideoFrameRef' to MRPT CImage and CMatrix in the case of Kinect.

By the way, I am newbie of GitHub.
This is my first pull request and very dirty. I apologize for this.
Sorry for my poor English.