fix(gles): Tidy locking in struct Buffer

[andyleiserson]

Fixes an issue with #8505.

Rather than use multiple adjacent mutexes and try to reason about protecting access to mapped: Cell<bool> with the GL context lock, puts all of the map state together under a single mutex, and removes the unsafe Send / Sync for struct Buffer.

Testing
Existing tests and keeping the compiler happy.

Squash or Rebase? Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[andyleiserson]

This is the key piece in all of this. Cell is not Sync and should not be used here.