Normalise non-EXIF orientation metadata when auto-orienting

[thedavidweng]

Problem

autoOrient() (and the related SetExifOrientation / RemoveExifOrientation helpers) only normalised the EXIF IFD0 Orientation tag. The same value is frequently duplicated in two other places — PNG text-chunk comments (e.g. files produced by macOS sips) and XMP packets (<tiff:Orientation>) — and these were left untouched.

After an auto-orient/bake-in operation the pixels are already rotated, but the stale PNG-comment / XMP orientation survives into the output. Consumers that honour XMP first (or read PNG text chunks) rotate the already-corrected pixels a second time, producing an incorrectly-displayed image.

Resolves #4585.

Solution

Add UpdateOrientationMetadata(), called from both SetExifOrientation and RemoveExifOrientation, which — alongside the existing EXIF IFD0 handling — also:

• updates PNG text-chunk comments whose keyword is EXIF:Orientation or xmp:tiff:Orientation (libvips exposes these as png-comment-<index>-<keyword> fields), and
• replaces every <tiff:Orientation>…</tiff:Orientation> element within an embedded XMP packet, tolerating attribute-bearing and self-closing forms and leaving values inside XML comments untouched.

Both writes take ownership of the buffer through glib-owned memory and a libvips free callback, so the metadata outlives the operation.

Explicit user-supplied metadata (e.g. .withXmp(...)) is intentionally left untouched, since it represents deliberate user input rather than inherited orientation.

Verification

• Reproduced against the released sharp@0.34.5: an input PNG carrying EXIF Orientation 6 plus an XMP <tiff:Orientation>6</tiff:Orientation> produced output whose EXIF Orientation was normalised to 1 but whose XMP orientation remained 6.
• After this change both the EXIF and the XMP/PNG-comment orientation values are normalised to 1, and the XMP packet survives intact (exiftool parses it without warnings).

Tests

Two unit tests in test/unit/rotate.js, each failing without the corresponding C++ change:

• Auto-orient normalises orientation duplicated in non-EXIF metadata — PNG text-chunk path (inputPngWithExifXmpOrientation.png).
• Auto-orient normalises XMP packet orientation and preserves packet integrity — real XMP packet path (inputPngWithXmpOrientation.png); also guards the buffer-lifetime fix.

npm run test-unit passes (the only failure locally — Lossless JPEG in test/unit/metadata.js — is pre-existing and unrelated; it reproduces on pristine main with my libvips build). npm run lint-cpp and biome lint are clean.

(Happy to add a changelog entry if you'd like — left it out since recent external-contributor PRs leave that to you at merge time.)