message: Implement get_tex_coord(sphere)
diff: #include "math/detail/math.h"
  #include "math/simd/type_traits.h"
  #include "math/array.h"
  +#include "math/constants.h"
  #include "math/sphere.h"
  #include "math/triangle.h"
  #include "math/vector.h"
  @@ -123,10 +124,17 @@ template <
  typename = typename std::enable_if<!simd::is_simd_vector<typename HR::scalar_type>::value>::type
  >
  VSNRAY_FUNC
  -inline auto get_tex_coord(HR const& hr, basic_sphere<T> /* */)
  +inline auto get_tex_coord(HR const& hr, basic_sphere<T> const& sphere)
  -> vector<2, typename HR::scalar_type>
  {
  - return {}; // TODO
  + using S = typename HR::scalar_type;
  +
  + auto n = (hr.isect_pos - sphere.center) / sphere.radius;
  +
  + return vector<2, S>(
  + atan2(n.x, n.z) / constants::two_pi<S>() + S(0.5),
  + n.y * S(0.5) + S(0.5)
  + );
  }
  