Repository: flutter
Issue: Populate_existing_damage doesn't work with multiple regions.
Labels: engine,e: embedder,P2,team-engine,triaged-engine

### Is there an existing issue for this?

- [X] I have searched the [existing issues](https://github.com/flutter/flutter/issues)
- [X] I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)

### Steps to reproduce

1. Use the Embedder API
2. implement `populate_existing_damage`(added in https://github.com/flutter/engine/pull/35022) and set multiple damage regions.

That seems to be a known problem, but there isn't an issue about it: https://github.com/flutter/engine/blob/d962fbb8aa22aaf3e2fcc7ccb17ad39131bc335c/shell/platform/embedder/embedder.cc#LL291C1-L291C1

### Expected results

To repaint all of the regions provided.

### Actual results

Repaints only the first provided region.

### Code sample

<details><summary>Code sample</summary>

A workaround solution:
Pass a single region containing all damaged regions.
```dart
void flutter_populate_existing_damage(void* user_data, intptr_t fbo_id, FlutterDamage* existing_damage) {
	ZenithServer* server = ZenithServer::instance();
	array_view<FlutterRect> damage_regions = server->output->swap_chain->get_damage_regions();

	// TODO: Who should free this object? Me or Flutter?
	// Also, I think Flutter's partial repaint mechanism is not completely implemented.
	// It only works with one rectangle. If I give it more than one, it just ignores them.
	// For this reason we just combine all damage regions into one rectangle.
	auto* union_region = new FlutterRect{};
	if (damage_regions.size() > 0) {
		*union_region = damage_regions[0];
		for (size_t i = 1; i < damage_regions.size(); i++) {
			*union_region = rect_union(*union_region, damage_regions[i]);
		}
	}

	existing_damage->struct_size = sizeof(FlutterDamage);
	existing_damage->num_rects = 1;
	existing_damage->damage = union_region;
}

// ... In another file, set as callback.
populate_existing_damage = flutter_populate_existing_damage;
```
Taken from: https://github.com/roscale/zenith/blob/9b17d5f03290daad237df8a72abb476886c5ce24/src/flutter_engine/embedder_callbacks.cpp#L164

</details>


### Screenshots or Video

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### Logs

<details><summary>Logs</summary>

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</details>


### Flutter Doctor output

<details><summary>Doctor output</summary>

```console
❯ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.10.0, on NixOS 23.11 (Tapir) 6.1.31, locale en_US.UTF-8)
[✗] Android toolchain - develop for Android devices
    ✗ Unable to locate Android SDK.
      Install Android Studio from: https://developer.android.com/studio/index.html
      On first launch it will assist you in installing the Android SDK components.
      (or visit https://flutter.dev/docs/get-started/install/linux#android-setup for detailed instructions).
      If the Android SDK has been installed to a custom location, please use
      `flutter config --android-sdk` to update to that location.

[✗] Chrome - develop for the web (Cannot find Chrome executable at google-chrome)
    ! Cannot find Chrome. Try setting CHROME_EXECUTABLE to a Chrome executable.
[✓] Linux toolchain - develop for Linux desktop
[!] Android Studio (not installed)
[✓] Connected device (1 available)
[✓] Network resources

! Doctor found issues in 3 categories.
```

</details>