Partial / Selective loading and instantiation

[atteneder]

Currently glTFast always loads the entire content of a glTF file before anything can be instantiated. It allows you to instantiate an individual glTF scene, but no more granular control.

It would be great to load and instantiate only a part of a larger file. That would decrease loading times and memory consumption.

Either by category (material, texture, animation) or knowing the inner structure of the file ahead, or between JSON parsing and loading binary loads.

Example use-cases:

• Load scene without
  • Materials
  • Textures
  • Animation
• Load only a certain material variant ( see #112 )
• Load only a certain LoD
• Load only a part of a scene's node tree (although I don't see a useful case here)

May be part of the bigger streamable refactor ( see #22 )