message: Extend primitive traits to basic_plane
diff: #include <cstddef>
  +#include <visionaray/math/plane.h>
  #include <visionaray/math/sphere.h>
  #include <visionaray/math/triangle.h>
  @@ -56,6 +57,12 @@ struct scalar_type
  // specializations ----------------------------------------
  +template <size_t Dim, typename T>
  +struct scalar_type<basic_plane<Dim, T>>
  +{
  + using type = T;
  +};
  +
  template <typename T>
  struct scalar_type<basic_sphere<T>>
  {
  