Ray/Sphere Intersection

Ray is defined by R(t) = Ro + Rd*t where t > 0

Ro = Origin of ray at (xo, yo, zo)
Rd = Direction of ray [xd, yd, zd] unit vector


Sphere's surface is defined by the set of points { ( xs, ys, sc)} where
(xs - xc)2 + (ys - yc)2 + (zs - zc)2 - Sr2 = 0

Sc = Sphere's center ( xc, yc, zc)
Sr = Sphere's radius