Refactoring SkyManager/Atmosphere

[gchoqueux]

SkyManager shows some architectural weaknesses and it needs to be refactor.

• Extend SkyManager from LayerGeometry and use LayerGeometry.update instead of scene.onBeforeRender
  ➡️ inherit layer features: events, visibility, freeze, add, delete, update loop
• Render pass management features in C3DEngine, which will facilitate future developments :
  • allowing users to add their own passes;
  • Eye-Dome-Lighting, depth picking.
• FXAAEffect could be replace by multi-pass rendering
• create a new pass for Sky
• remove this.scene, this.scene.onBeforeRender, this.scene.onAfterRender
  ➡️ use Layer.preUpdate and Layer.postUpdate
• use Layer.object3d to add all object 3D in SkyManager
• remove getter and setter enabled, enable() and disable() that are redundant with each other and
  ➡️ use Layer.visible

[Neptilo]

Also to do as part of the refactoring: make the internal properties private readonly and prepend _ to their names.