Add a loading state/screen

[Malayvasa]

In the current pipeline, the hdri loads first, then the character and then the scene. Let's display all of it together once loaded.

[gv-sh]

Comment for self: Implement asynchronous loading for scene initiation.

[gv-sh]

In view of time, I have implemented a time out of 3 seconds which gives adequate time to build the scene in the background. I will map this to reflect actual loading of the scene components.