Adjust the placement of the zero size check for some texture copies

[andyleiserson]

It is important that we not call handle_*_init if we are not actually going to perform a transfer.

The order of operations is changed for copy_texture_to_texture and copy_external_image_to_texture. The rest are just adding the comment.

Testing
Tested manually in Firefox.

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.