Add breaking change notice for OpenGL ES render-to-texture orientation

[bdero]

Description of what this PR is changing or adding, and why:

Adds a breaking change notice for the Impeller OpenGL ES change in flutter/flutter#186556, which stores render-to-texture content top-down to match Metal and Vulkan. The only surface that can observe the old orientation is the experimental Flutter GPU API, so this is a courtesy notice rather than a hard breaking change.

Issues fixed by this PR (if any):

None.

PRs or commits this PR depends on (if any):

flutter/flutter#186556. The Timeline version is a placeholder until that engine PR lands and the release is known.

Presubmit checklist

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• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks of 80 characters or fewer.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit dd7d1d2):

https://flutter-docs-prod--docs-pr13419-bdero-breaking-change-bys51mj7.web.app

[gemini-code-assist]

Code Review

This pull request documents a breaking change where Impeller's OpenGL ES backend now stores render-to-texture content top-down, aligning it with Metal and Vulkan. It includes a new breaking change document and updates the index. I have no feedback to provide.

[gaaclarke]

This is worded too specifically for Flutter GPU, it has broader impact.

[gaaclarke]

This needs to be updated to reflect that it affects Flutter users using the FragmentShader API, not just Flutter GPU users.

[gaaclarke]

Let's include the whole preprocessor conditional compilation so people have a better idea of what they'll be looking for.