Support native multi-platform non-blocking texture loading

[Texture2D.LoadImage](https://docs.unity3d.com/ScriptReference/Texture2D.LoadImage.html) is slow, can only be ran on the main thread, and blocks the renderer while loading. This is a dealbreaker for a lot of projects, especially those involving VR as dropping frames causes motion sickness.

Start with Unity's Texture2D.LoadImage documentation, which the issue identifies as the current loading API. Work out a native, cross-platform approach that does not require the main thread or block the renderer; completion should address the frame drops that are especially problematic for VR.

Written by the indexing model from the issue text.