Error using keepGainMap() + keepMetadata()

@lovell Great to see [keepGainMap()](https://github.com/lovell/sharp/issues/4314#issuecomment-4191094196) support added! I'm doing some testing with **0.35.0-rc.2** and see a potential bug: ## Summary On my local setup, `keepGainMap()` and `keepMetadata()` each work on their own, but chaining them as: ```js sharp(input).keepGainMap().keepMetadata().jpeg({ quality: 90 }) ``` fails for gain-map JPEG input with: ```txt vips_image_get: field "gainmap-max-content-boost" not found ``` This appears to be specific to the `keepGainMap()` + `keepMetadata()` combination on gain-map JPEGs, not a general `keepMetadata()` failure. ## Environment source images: [sample gain maps.zip](https://github.com/user-attachments/files/26577192/sample.gain.maps.zip) - Node.js v20.12.2 - `sharp` 0.35.0-rc.2 - libvips 8.18.2 ### Example ```js import sharp from "sharp"; const input = "myGainMap.jpg"; await sharp(input) .keepGainMap() .keepMetadata() .jpeg({ quality: 90 }) .toBuffer(); ``` ### Actual result ```txt Error: vips_image_get: field "gainmap-max-content-boost" not found ``` ### Expected result The transcode should succeed and preserve both: - the gain map via `keepGainMap()` - metadata via `keepMetadata()` ## Control cases These all succeeded locally with the same input: ```js await sharp(input) .keepMetadata() .jpeg({ quality: 90 }) .toBuffer(); await sharp(input) .keepGainMap() .jpeg({ quality: 90 }) .toBuffer(); await sharp(input) .withGainMap() .keepMetadata() .jpeg({ quality: 90 }) .toBuffer(); await sharp(input) .keepGainMap() .keepExif() .keepIccProfile() .keepXmp() .jpeg({ quality: 90 }) .toBuffer(); ``` This also succeeded, which suggests the issue may be ordering-related: ```js await sharp(input) .keepMetadata() .keepGainMap() .jpeg({ quality: 90 }) .toBuffer(); ```