async implementation

this build has the following content:
 1) async implementation for glb/gltf files and sketchfab zip files
 2) tons of bugfixes
 3) it now reduces 1024x1024, 2048x2048, 4096x4096 and 8192x8192 sized textures to 512x512 to improve loading performance
     and memory consumption. We couldn't see any difference in rendering quality.
 4) there's progress bar config feature, which allows modifying progress bar behaviour

That's all folks!