Reintroduce buffer snatching Part 1

[nical]

Connections

This is the first step towards addressing #4787.

Description

We need destroy() methods to be able to deallocate gpu resources while the associated handle is still alive. This is difficult to do safely because most of wegpu's API can be used from any thread.One of the steps towards our goal is to re-introduce "buffer snatching". The idea is that when, say, buffer.destroy() is called, we snatch the raw buffer out of wgpu_core's buffer struct, and enqueue it to be deallocated after the latest submission the buffer has been used in is complete.
Snatching the raw handle out requires some form of locking. @cwfitzgerald's proposed solution is to protect all raw buffer and texture reads behind the read guard of a device-global RwLock, and only ever thake a write lock during snatching. The hope is that the overhead of taking a single read guard in most places will be small enough and that destroy will be called concurrently with other wgpu work rarely enough that it won't cause very bad contention.

This PR implements the very first step of Connor's idea:

• Device holds snatchable_lock.
• a Snatchable<T> type is introduced. Its content cannot be accessed without an appropriate lock guard.
• Buffer.raw is now a ``Snatchable<A::Buffer>` and all associated reads need the guard.

The snatching is not actually performed in this PR (yet). It will come in a followup.

Checklist

• Run cargo fmt.
• Run cargo clippy.
• Run cargo xtask test to run tests.

[nical]

I don't want this marked as draft, it should get reviewed and hopefully landed. The "WIP" in the title was referring to the fact that it's a part of a larger ongoing refactoring, I'll remove it since it is more confusing than helpful.

[cwfitzgerald]

Looks good! Some organizational notes.

[ErichDonGubler]

I don't want this marked as draft, it should get reviewed and hopefully landed. The "WIP" in the title was referring to the fact that it's a part of a larger ongoing refactoring, I'll remove it since it is more confusing than helpful.

Ah, sorry!

[cwfitzgerald]

Nice!