Load Time Optimization: Faster Environment Map loading

[vilbeyli]

Currently, engine doesn't return from load function untill all environment maps are loaded.

They require rendering to cubemaps and they take time generate.

Engine could load the current one that is set in the level, and then return from the load function while queueing the remaining environment maps to a background loading thread. This way we can laod the scene and can wait on the environment map if the user decides to change it right after loading.