Implement environment mapping

[Adnn]

• [x] Load a cubemap
  • [x] Show as a skybox
  • [x] Mipmap the equirectangle: gives bad results!!
• [x] Provide conversion from equirectangular to cubemap
• [x] IBL
  • [x] Specular
  • [x] Implement Brian Karis split-sum approximation from Real Shading in Unreal Engine 4
    • [x] Enhance filtering with mipmaps Gpu Gems 3 ch20
    • [x] Compare result form learnopengl mipmap selection
    • [x] rtr 4th multiply 1st term with D, try it: gives bad results, added a "todo" and comments
    • [x] The mipmap level selection of the prefiltered environment should take into account the area covered by the screen pixel (p418)
  • [x] Skew normal (rtr 4th p422: Lagarde [960] suggests skewing the vector used to fetch the prefiltered environment map)
  • [x] Diffuse
  • [x] Add a Fresnel term
• [ ] Support a container format (ktx2, dds)
  • [ ] Compressed textures
  • [ ] Mipmaps directly from the container