Changes:
- Track whether an output texture has been written to yet and only clear
it on the first write.
- Use `ClearColorConfig` on `CameraOutputMode` instead of a raw
`LoadOp`.
- Track whether a output texture has been seen when specializing the
upscaling pipeline and use alpha blending for extra cameras rendering to
that texture that do not specify an explicit blend mode.
Fixes#6754
## Testing
Tested against provided test case in issue:
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## Changelog
- Allow cameras rendering to the same output texture with mixed hdr to
work correctly.
## Migration Guide
- - Change `CameraOutputMode` to use `ClearColorConfig` instead of
`LoadOp`.
027f8e21ec