ClearColorConfig not honored with multiple cameras

## Bevy version and features

0.20.0-dev (mainline)

## \[Optional\] Relevant system information

- MacOS 15.7.4

```ignore
AdapterInfo { name: "Apple M1 Pro", vendor: 0, device: 0, device_type: IntegratedGpu, device_pci_bus_id: "", driver: "", driver_info: "", backend: Metal, subgroup_min_size: 4, subgroup_max_size: 64, transient_saves_memory: Some(true), limit_bucket: None }
```
## What you did

Three Camera2ds sharing a window.

## What went wrong

Only the last one's ClearColorConfig honored and it clears the whole window instead of its own Viewport

## Additional information

Example reproduction:

[repro_camera_bug.rs.zip](https://github.com/user-attachments/files/31629449/repro_camera_bug.rs.zip)

Start by running the attached repro_camera_bug.rs.zip with Bevy 0.20.0-dev and observe the three Camera2d instances sharing a window. Trace how ClearColorConfig is applied when multiple cameras use separate viewports. Done means each camera's clear color affects only its own viewport rather than the last camera clearing the whole window.

Written by the indexing model from the issue text.