[web] Decode images once in instantiateImageCodecWithSize - #196029

On the web, instantiateImageCodecWithSize with a getTargetSize callback (the path ResizeImage, and so Image.network(cacheWidth: ...), goes through) decoded every image twice. It created a codec, decoded a full-size first frame into a ui.Image only to read width and height, disposed it, and then created a second codec at the target size. On Chromium, the full-size frame was also copied into a full-size ImageBitmap before upload, so a resized image still allocated its full-size bitmap for a moment.

This PR adds engineInstantiateImageCodecWithSize. When ImageDecoder is available, it creates one BrowserImageDecoder, decodes the first frame, passes its size to getTargetSize, and returns a codec at the target size that reuses that decoder. The decoded first frame is kept by the new BrowserImageDecoder.decodeFirstFrameSize and returned by the next getNextFrame, so it is not decoded again. Later frames of animated images are decoded in order as before. A kept frame is closed if the decoder is disposed first. Without ImageDecoder, the behavior is unchanged (decode once for the size, then again at the target size).

Measurements in Chrome 154 on an Apple Silicon Mac, with a script that makes the same browser calls as the old and new paths (ImageDecoder setup and decode, createImageBitmap, resize to 400 px wide), median of 7 runs:

Tests:

• image_test.dart: instantiateImageCodecWithSize creates no ui.Image before getNextFrame, and exactly one after it.
• image_decoder_test.dart: decodeFirstFrameSize keeps the first frame for getNextFrame; dispose closes a kept frame; an animated GIF decoded through instantiateImageCodecWithSize returns the same number of frames, with the same durations in the same order, as the unsized codec.
• The existing instantiateImageCodecWithSize tests (buffer disposal, temporary image disposal, upscaling) cover the unchanged behavior.

dart analyze and dart format pass on the changed files. felt test was not run locally, because there is no full engine checkout here.

Fixes #118985

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.