Add GLUT include directory to OpenNIVisualTest + Fix for double to float
conversion warning + Change uint (unrecognized) to unsigned int.
Add comment to OpenNI doxygen page about PATH

On Windows, you need to add the OpenNI2 Redist directory to your PATH in
order for the VisualTest and UnitTest to find the OpenNI dll's. This
change adds a comment to the OpenNI documentation about this.
In the OpenNI device rename "image" to "color"

Renamed the images data group entry from "image" to "color" for the rgb
color image provided by the OpenNI device. This makes the distinction
more clear between the other "depth" image.

Also, clarified a couple of comments about converting OpenNI specific
image values to OSS values.