ray_sphere_intersect [function]
ray_sphere_intersect( vec3 ray_pos, vec3 ray_dir, vec3 sphere_pos, real radius )
tests for an intersection between ray and sphere, returning the distance
- if ray does not intersect sphere, false is returned
- otherwise, signed distance between ray and sphere is returned
- if signed distance along ray to sphere is larger than 0, sphere is in front of ray