Fix msaa writeback prevents clear color

[beicause]

Objective

Fixes #22453, extracted from #25226

Solution

Only run msaa writeback if camera clear color config is None.

It's OK because camera with LoadOp::Clear will clear the entire main texture thus writeback is useless.

Testing

Run the example in #22453

[stuartparmenter]

This looks right to me. Writeback only matters when the camera has ClearColorConfig::None, my check in #25479 was doing this the complicated way. I was looking at this further after that PR, but hadn't had a chance to file an issue or PR yet --

Can you look at these 3 things?

1. Don't call mark_as_cleared on the destination in the writeback. Nothing depends on it, and it's what causes the camera's clear to turn into a load. Removing it also fixes MsaaWriteback::Always, which drops the clear color today.
2. Fix the examples relying on the old behavior: first_person_view_model, loading_screen, and ui_target_camera. Each has a fullscreen camera on top with a default clear color, so with this change the top camera hides everything below it. Set ClearColorConfig::None on those cameras, and remove the comment in ui_target_camera that says only the lowest camera clears.
3. Add a migration guide

[beicause]

I made MSAA writeback never run if camera is not ClearColorConfig::None so we don't need to avoid ViewTarget::mark_as_cleared

loading_screen doesn't need ClearColorConfig::None because it draws a fullscreen black Node.

[stuartparmenter]

I made MSAA writeback never run if camera is not ClearColorConfig::None so we don't need to avoid ViewTarget::mark_as_cleared

loading_screen doesn't need ClearColorConfig::None because it draws a fullscreen black Node.

Thanks, gating on clear_color == None is a fine way to handle mark_as_cleared, works for me.

A few things left:

1. loading_screen still needs ClearColorConfig::None on the UI camera. The fullscreen black node gets hidden once the level is ready, but the camera keeps rendering with the default clear color and wipes the 3D scene every frame. It only works today because writeback was turning that clear into a load.
2. I don't think your change from Auto being clear_color == None to sorted_camera_index_for_target > 0 && clear_color == None is right. It undoes part of Make ClearColorConfig::None work correctly (esp in the presence of MSAA) #23844 where a single camera with None and MSAA on no longer writes back, so it loses what it rendered last frame. Since clear_color == None is already required for any writeback, Auto can just be true. The Auto docs and the comment in prepare_msaa_writeback_pipelines say the first camera has nothing to write back, which isn't right either.
3. ui_target_camera: I'd just drop the "cleared according to the camera with the lowest order" comment. The engine doesn't do that, it only looks that way because the observer reassigns every camera's clear color.

[stuartparmenter]

1. For the loading screen, the UI camera does not need any 3D scene content at all; in this case, LoadOp::Clear will have better performance than LoadOp::Load which is unnecessary.

I ran loading_screen on your branch and after the level loads I get a blank grey screen instead of the fox I see on main. The UI camera doesn't read the 3D content, but it's drawn into the same main texture and then alpha-blended over the window. The default clear color is opaque, so the clear covers the scene. Setting ClearColorConfig::None on that camera brings the fox back.

2. "a single camera with None and MSAA on no longer writes back" is exactly the difference between Auto and Always.

Auto skipping the first camera was the behavior before #23844. It seems like that PR added the None case to Auto because a single camera with ClearColorConfig::None and MSAA was throwing away its main texture, so your change would be reverting that fix. I'd rather Auto stay correct by default and Always be the odd one out, but maybe @tychedelia has an opinion here?