message: Add a reference for apex-less formula derivation
  Note that we use a simplified more approximate version that doesn't
  require inverse trigonometry by replacing sin(a+b) with a more
  conservative sin(a)+sin(b).
diff: @@ -501,7 +501,8 @@ struct meshopt_Bounds
  * dot(center - camera_position, cone_axis) >= cone_cutoff * length(center - camera_position) + radius
  *
  * The formula that uses the apex is slightly more accurate but needs the apex; if you are already using bounding sphere
  - * to do frustum/occlusion culling, the formula that doesn't use the apex may be preferable.
  + * to do frustum/occlusion culling, the formula that doesn't use the apex may be preferable (for derivation see
  + * Real-Time Rendering 4th Edition, section 19.3).
  *
  * vertex_positions should have float3 position in the first 12 bytes of each vertex
  * index_count/3 should be less than or equal to 512 (the function assumes clusters of limited size)
  