add freenect_map_rgb_to_depth helper function (untested)
clear rgb pixels without depth data for map_rgb_to_depth
Adds zbuffer to the mapping of rgb to depth

Signed-off-by: Francisco Facioni <fran6co@gmail.com>
Merge pull request #395 from fran6co/rgb2depth

Add RGB -> depth mapping

Reviewed-by: Benn Snyder <benn.snyder@gmail.com>