Free command encoders' platform resources on drop on _all_ platforms

[ErichDonGubler]

Connections
Link to the issues addressed by this PR, or dependent PRs in other repositories

This is my attempt to investigate and resolve #3193. I believe this code is ready to go, but have not yet confirmed internal details to my satisfaction. I'm leaving this as WIP for now because I will be unavailable to work on finalizing details until ~2024-02-21. If somebody wants to pick it up before then, great!

Description
Describe what problem this is solving, and how it's solved.

I believe that #3193 is caused by not freeing command encoders early enough. This is apparent on the DX12 backend, where the DX12 API actually emits an error to inform us of our mistake. This is likely causing issues in other backends (i.e., Vulkan), where the symptoms aren't as obvious. Metal has already been cared for by @bradwerth with #2077, so only the other backends need attention here.

Interestingly, we don't currently do any clean-up of command encoders when they are dropped, but we do when they're finished. This seems wrong. So, this change adds a CommandEncoder::discard_encoding call for command encoders within wgpu_core::Global::command_encoder_drop. This change also adds a similar call to the Drop implementation of backends that appear to be idempotent in their CommandEncoder::discard_encoder calls (N.B., this does not include Vulkan). This appears to resolve DX12 API errors.

To be completely clear about how each backend is changed here:

Backend	Status before this PR	Status after this PR
Vulkan	Possibly has issues	✅Fixed.
DX12	Reporting hard errors	✅No more errors!
Metal	Already handled by #2077.	-
GLES	Possibly has issues	✅Fixed.

Testing
Explain how this change is tested.

A test (wgpu_test::buffer::command_encoder_without_finish_fine) has been added, demonstrating both the expected failure before the fix and resolution to "just" passing.

This change changes numerous expected error cases for tests for DX12, most notably wgpu_tests::encoder::drop_encoder_after_error, to simply be passing.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[torokati44]

anxiously waiting for a release with this fix in, preferably in the 0.19 series
https://github.com/ruffle-rs/ruffle/actions/runs/8046091966/job/21972653869

[ErichDonGubler]

@torokati44: Have you confirmed that this fix addresses an issue in Ruffle? It's unclear if the CI issue you're linking is caused by this fix, or fixed by this fix. 😅

[torokati44]

Since Ruffle is on v0.19.1, which was released before this got merged, the CI issue is definitely not caused by this PR, but hopefully fixed by it.
And, no, haven't verified it yet, but I'd give it more than 60% chance, just by gut feeling. 😶

[cwfitzgerald]

Agh - I honestly thought this PR was caught up in all the other internal work, I didn't realize it was so backportable. There will be a 0.19.3 decently soon, so I'll include this in that