* Remove `Arc` around `RenderPipeline`
It is not necessary anymore in last `wgpu` version.
* Replace `RwLock` with `Mutex`
It was only used for writing.
* Support sharing `Pipeline` state between `TextAtlas`
* Keep using `Vec` for pipeline cache
* Use `OnceCell` to keep `Pipeline` private
* Revert "Use `OnceCell` to keep `Pipeline` private"
This reverts commit 4112732b1734a3bb6b915d2103e699ef549b77c1.
* Rename `Pipeline` type to `Cache`
