Check the math for buffer-texture copies

[andyleiserson]

Check the math for buffer-texture copies (i.e. validate_linear_texture_data) more carefully. This fixes the original issue reported in #1039, but not the other cases I mentioned in a comment.

Testing
Enables a CTS test and adds directed tests.

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.