Rewrite screenshots.

[tychedelia]

Objective

Rewrite screenshotting to be able to accept any RenderTarget.

Closes #12478

Solution

Previously, screenshotting relied on setting a variety of state on the requested window. When extracted, the window's swap_chain_texture_view property would be swapped out with a texture_view created that frame for the screenshot pipeline to write back to the cpu.

Besides being tightly coupled to window in a way that prevented screenshotting other render targets, this approach had the drawback of relying on the implicit state of swap_chain_texture_view being returned from a NormalizedRenderTarget when view targets were prepared. Because property is set every frame for windows, that wasn't a problem, but poses a problem for render target images. Namely, to do the equivalent trick, we'd have to replace the GpuImage's texture view, and somehow restore it later.

As such, this PR creates a new prepare_view_textures system which runs before prepare_view_targets that allows a new prepare_screenshots system to be sandwiched between and overwrite the render targets texture view if a screenshot has been requested that frame for the given target.

Additionally, screenshotting itself has been changed to use a component + observer pattern. We now spawn a Screenshot component into the world, whose lifetime is tracked with a series of marker components. When the screenshot is read back to the CPU, we send the image over a channel back to the main world where an observer fires on the screenshot entity before being despawned the next frame. This allows the user to access resources in their save callback that might be useful (e.g. uploading the screenshot over the network, etc.).

Testing

TODO:

• Web
• Manual texture view

---

Showcase

render to texture example:

web saving still works:

Migration Guide

ScreenshotManager has been removed. To take a screenshot, spawn a Screenshot entity with the specified render target and provide an observer targeting the ScreenshotCaptured event. See the window/screenshot example to see an example.

[killercup]

I've tried to follow the reasoning and the change and they make sense to me. I'm not confident in my skills to review the render changes.

[tychedelia]

I'm not totally sure if we should rewrite the headless renderer example to use this. It certainly would reduce a lot of boilerplate in that example but the example is also more efficient since it doesn't recreate textures and buffers for every screenshot like we do here. We could make this more efficient in engine by keeping those resources around rather than creating them for every new screenshot, but I'm not totally sure whether that's a use case we actually want to support in engine...

[killercup]

Example works, and code makes sense. Left a couple inline comments but consider this approved :)

[NthTensor]

This PR is well motivated and I much prefer the new component/observer api. I am a little wary of the changes to prepare_view_textures/targets, they seem generally like an improvement but are not quite 1-to-1 with the behavior of the original.

In terms of reusing textures and porting over the headless example, I think that's something we can investigate as follow up. The perf and ergonomics are fine for now, and the screenshot example demonstrates use well enough.

[NthTensor]

Yep, looks like this preserves the old behavior now. Haven't run this code yet, but approving anyway.

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1678 if you'd like to help out.