Mapping shadow resources forces thread sync in GPU driver

[TellowKrinkle]

Ryza really likes to update certain vertex buffers with a copy to a staging buffer, map READ_WRITE, unmap, copy back to vertex buffer. While the shadow buffers prevent this from becoming a full GPU sync, the map still causes a sync with internal gpu driver threads on both DXVK and the Windows AMD driver. I attempted a horrible hack to avoid this (28b19cc7358ee9ca274cd9cbe424bb99ea3716b6), and it turns out that in addition to letting me finally run my synthesis UI at 144fps, it also clears up a fairly bad fps drop that you get in Ryza 2 every time you jump on a Steam Deck. (Sadly that hack is so hacky it doesn't even manage to work on all Atelier games, breaking Sophie DX.)

So if anyone has a less-hacky solution to this, I'd be very interested.