Parallelize prepare_assets::<T> systems

[brianreavis]

Objective

Because prepare_assets::<T> had a mutable reference to the RenderAssetBytesPerFrame resource, no render asset preparation could happen in parallel. This PR fixes this by using an AtomicUsize to count bytes written (if there's a limit in place), so that the system doesn't need mutable access.

• Related: Throttle render assets #12622

Before

After

Testing

• Tested on a local project (with and without limiting enabled)
• Someone with more knowledge of wgpu/underlying driver guts should confirm that this doesn't actually bite us by introducing contention (i.e. if buffer writing really should be serial).