Improve thumbnail loading performance

[oreyg]

Current implementation uses WebRequestManager to load thumbnail. It returns byte array, which is then passed to Texture2D.Load. This is a blocking function, that does its work on the main thread, and thus creates FPS spikes. This is especially noticeable on mobile devices. Instead 'UnityWebRequestTexture' should be used.

[btco]

That's a great point. We use WebRequestManager because of caching, but it would be much better to use UnityWebRequestTexture.