Apply EXIF orientation to decoded images; render image documents in iframes

[nicoburns]

Summary

Two small changes in blitz-dom/src/net.rs that take css/css-images/image-orientation WPT from 14/34 to 21/34 passing.

EXIF orientation (image-orientation: from-image, the initial value). ImageHandler::parse previously used ImageReader::decode(), which discards the decoder's orientation metadata, so JPEGs/PNGs with EXIF rotation/flip rendered un-rotated. Now:

let mut decoder = ImageReader::new(..).with_guessed_format()?.into_decoder()?;
let orientation = decoder.orientation().unwrap_or(Orientation::NoTransforms);
let mut image = DynamicImage::from_decoder(decoder)?;
image.apply_orientation(orientation);

The transform is baked in at decode time (width/height in Resource::Image are post-rotation), so everything downstream (<img>, background-image, border-image, list-style-image, mask-image, content:) picks it up for free.

Image documents in <iframe>. DocumentSrcHandler treated every response as HTML, so an iframe whose src is a JPEG rendered the raw bytes as text. If image::guess_format recognises the bytes, we now synthesise a minimal <img src="{url}"> document instead (the image is then fetched via the normal image path, so it gets EXIF handling too).

Remaining failures (13) — not addressed here

• 10 need image-orientation: none: background-image, none, none-content-images, *-dynamic2, *-computed-style, none-cross-origin*. Stylo declares image-orientation with engine = "gecko" in properties/longhands.toml, so the property isn't generated for the Servo build and can't be parsed by Blitz at all. Supporting it needs an upstream Stylo change plus keeping the un-oriented pixels around (or applying the orientation as a paint-time transform instead of at decode). The cross-origin variants additionally need multi-origin serving and <canvas>.
• exif-png.html: EXIF stored in a legacy compressed zTXt "Raw profile type exif" chunk. No browser passes this on wpt.fyi either.
• background-properties(-border-radius): renders visually identical, but JPEG decode noise between the EXIF file and the pre-rotated reference file gives 406 differing pixels (max diff 2) vs the fuzzy budget of 313/331. Chrome/Edge also fail one or both of these.

Side finding: new URL(img.src) throws "relative URL without a base" in the cross-origin tests — the HTMLImageElement.src getter returns the raw attribute rather than the resolved absolute URL.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/67f3ef53d8b744548190d4a3dfa8f9dc
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/67f3ef53d8b744548190d4a3dfa8f9dc?variant=devin-insiders
Requested by: @nicoburns

WPT results

11 newly passing, 4 newly failing (net +7).

Full diff (15 changed tests)

+ Fail => Pass css/css-images/image-orientation/image-orientation-background-position.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-default.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-exif-png-2.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-exif-png-3.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-from-image-composited-dynamic1.html
- Pass => Fail css/css-images/image-orientation/image-orientation-from-image-composited-dynamic2.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-from-image-composited.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-from-image-content-images.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-from-image-dynamic1.html
- Pass => Fail css/css-images/image-orientation/image-orientation-from-image-dynamic2.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-from-image-embedded-content.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-from-image.html
- Pass => Fail css/css-images/image-orientation/image-orientation-none-content-images.html
+ Fail => Pass css/css-images/image-orientation/image-orientation-none-image-document.html
- Pass => Fail css/css-images/image-orientation/image-orientation-none.html

_{Generated by the WPT workflow.}

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