Load models in background thread

[375gnu]

Celestia itself is a single threaded application, so when an object which uses models comes to viewport Celestia stutters for a while because it loads that model and its textures. The problem is that this model may never become visible and so users unfamiliar with Celestia may treat it as unusable due to stutters without any reason.

Today even the cheapest mobile CPUs have several cores so it would be good to load models and textures in a background thread to reduce stutters.

[levinli303]

this might also help make it possible to host celestia content on the internet in the future. since network request should always happen on other threads.

maybe we can render a placeholder model/texture as the loading is underway?