Make wgpu-core's copy_buffer_to_buffer size parameter optional

[andyleiserson]

This makes the size parameter to wgpu-core's copy_buffer_to_buffer implementation optional. When the size is omitted, the copy spans from the source offset to the end of the source buffer.

The goal here is to make the size parameter to copyBufferToBuffer optional in Firefox. https://bugzilla.mozilla.org/show_bug.cgi?id=1959728. We might be able to achieve this without changing the signature of wgpu copy_buffer_to_buffer functions at all, but note that trace::Command::CopyBufferToBuffer is used to serialize the command between the content process and GPU process, so keeping size non-optional in that enum variant would significantly complicate things in Firefox.

I didn't change the signature of copy_buffer_to_buffer on wgpu (non-core). That seemed like a fairly disruptive breaking change.

Testing
No tests, right now, other than not breaking existing tests. This is covered by the WebGPU CTS although that's not currently running in the wgpu repo.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[crowlKats]

this shouldnt be like this, but rather the paramater of the function calling this should have its type be made Option<u64>