OpenNI2: prevent failure of setVideoMode() call with invalid parameters

Not all parameters are specified for openni::VideoMode, so
"selected" mode can be unsupported for device.
Replace default VideoMode constructor to result of getVideoMode() call.
openni_capture sample: enable OpenNI2 if available

Sample works with OpenNI2 with minimal code changes (CAP_OPENNI -> CAP_OPENNI2)
Merge pull request #3404 from alalek:openni2_patch