Approximation for columnar density ratio encountered by light ray over a planet

"I" is the integral we want to find. We're trying to sum up the density of air encountered along a path. Density varies as exp(-h) with height (h) and height is our distance to the center of the earth (sqrt(x^2+z^2)) minus the planet's radius (R), so we're trying to find the integral of that. I returns the equivalent distance that would need to be traveled along the surface of the planet in order to encounter that same mass of air. As always, all units are in scale heights.

"C" (short for "Chapman") is the Chapman function when looking parallel to the ground, as described by Schüler (http://www.gameenginegems.net/gemsdb/article.php?id=1133). It's been slightly modified to work with our approximation