feat: allow providing background color when transforming images with alpha channel to a format without it

[szuperaz]

🎯 Goal

Feature request: https://getstream.slack.com/archives/C02GBL5M1BK/p1788176084433019

The problem:

• When transforming images with an alpha channel (PNG/Webp) to a format without an alpha channel (for example JPEG) the resulting image's background is platform-dependent: black on Android, white on iOS 26
• The request: provide an option for integrators to specify an explicit background color for these transforms

How to use it?

const localCompressImage = defaultNativeHandlers.compressImage;

registerNativeHandlers({
  compressImage: localCompressImage
    ? (params) => localCompressImage({ ...params, backgroundColor: '#FFFFFF' })
    : undefined,
});

🛠 Implementation details

Works for CLI-only because expo-image-manipulator doesn't have such option. The default background color is white.

🎨 UI Changes

iOS

Before	After

Android

Before	After

🧪 Testing

☑️ Checklist

• I have signed the Stream CLA (required)
• PR targets the develop branch
• Documentation is updated
• New code is tested in main example apps, including all possible scenarios
  • SampleApp iOS and Android
  • Expo iOS and Android

[Stream-SDK-Bot]

SDK Size

title	develop	branch	diff	status
js_bundle_size	2031 KB	2031 KB	0 B	🟢

[isekovanic]

Other than the couple of comments, looks good, thank you !

I'm wondering if we should add some default background color but I guess that's breaking, so we have to sort of stick to what we have currently I believe.

[stream-ci-bot]

🎉 This PR is included in version 9.9.0 🎉

The release is available on:

• v9.9.0
• GitHub release

Your semantic-release bot 📦🚀