Merge of lp:~glmark2-dev/glmark2/refract

Adds new SceneRefract, which performs a two interface refraction through an
object, as well as a surface reflection.  The object (model), environment
(texture), and index of refraction of the object medium (index) are all
adjustable via command line options, but the default is to render the Stanford
Bunny against a background of the 'nasa1' texture with a refractive index of
1.2; the medium surrounding the object is assumed to be air/vacuum (index==1.0).