[bugfix] Publish reasonable camera info when changing depth resolution.

Fixes multiple problems:

- when depth registration was enabled, the camera info would always be for the rgb image, even when this has a different resolution
- when the kinect is calibrated, it would always publish the calibrated camera info even if resolutions don't match
- The principal point would be modified for the depth image based on a 640x480 image, no matter what the actual resolution was

Also, the logic in getDepthCameraInfo is a little less obscure now,; it's behaviour doesn't change.