Neos VR update for 28 December 2019

Another smallish build. Updated Assimp native importer library to latest, added arbitrary UV channel and 3D/4D UV support to internal mesh format.

Tweaks:

• Added support for arbitrary number of UV channels for the MeshX (Neos' internal mesh format)
  -- This is futureproofing the system. Currently up to 8 channels will be uploaded to the GPU
  -- Model importer will import all channels that it finds
• Added support for 3D and 4D UV channels, in addition to standard 2D
  -- This allows packing extra data into the meshes. This is necessary for certain TiltBrush brushes that @0utsider is working on
• Updated Assimp importer library to latest version from source (over 300 commits)
  -- This has numerous improvements and fixes, including improved FBX compatibility (e.g. some more PBR textures should now import)
• Added protections against deleting or moving some crucial slots in the world that would cause issues (based on feedback by @Shifty and @RueShejn)