Fix buffer offset corruption in runtime image decoders

[Copilot]

Runtime decoders (ImageDecoder/OffscreenCanvas) were corrupting pixel data when creating views from Uint8ClampedArray buffers with non-zero offsets.

Problem

Decoding via runtime APIs produced scrambled images on re-encode:

const bytes = await Deno.readFile("image.jpg");
const img = await Image.decode(bytes, "jpeg");
const reencoded = await img.encode("jpeg");
// Result: visibly corrupted pixel data

The decoders assumed imageData.data started at buffer offset 0:

// BROKEN: ignores byteOffset
return new Uint8Array(imageData.data.buffer);

Changes

• JPEG, WebP, GIF, TIFF, HEIC, AVIF decoders: Changed buffer access to properly copy pixel data regardless of offset

  // FIXED: respects byteOffset and byteLength
  return new Uint8Array(imageData.data);

• Tests: Added roundtrip validation for lossless (PNG) and lossy (JPEG) formats with pixel-level verification

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• github-production-user-asset-6210df.s3.amazonaws.com
  • Triggering command: /usr/bin/curl curl -L -o A.jpg REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Instances of pixel-offset corruption after decoding/re-encoding</issue_title>
<issue_description>Hello! Thank you for this very useful tool.

I've discovered a couple of instances of image distortion when simply decoding and then re-encoding in the same format. I've only tested in Deno (v2.6.4).

To reproduce, simply drop the attached files into a directory and then run the following script with all permissions:

import { Image } from "@cross/image";
import * as path from "@std/path";

const aBytes = await Deno.readFile(path.join(Deno.cwd(), "A.jpg"));
const aImg = await Image.decode(aBytes, "jpeg");
const aImgPrime = await aImg.encode("jpeg");
await Deno.writeFile(path.join(Deno.cwd(), "aPrime.jpg"), aImgPrime);

const bBytes = await Deno.readFile(path.join(Deno.cwd(), "B.png"));
const bImg = await Image.decode(bBytes, "png");
const bImgPrime = await bImg.encode("png");
await Deno.writeFile(path.join(Deno.cwd(), "bPrime.png"), bImgPrime);

Really hoping you can help ...</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Instances of pixel-offset corruption after decoding/re-encoding #94

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