Take example screenshots in CI

[mockersf]

Objective

• I want to take screenshots of examples in CI to help with validation of changes

Solution

• Can override how much time is updated per frame
• Can specify on which frame to take a screenshots
• Save screenshots in CI

I reused the TimeUpdateStrategy::ManualDuration to be able to set the time update strategy to a fixed duration every frame. Its previous meaning didn't make much sense to me. This change makes it possible to have screenshots that are exactly the same across runs.

If this gets merged, I'll add visual comparison of screenshots between runs to ensure nothing gets broken

Migration Guide

• TimeUpdateStrategy::ManualDuration meaning has changed. Instead of setting time to Instant::now() plus the given duration, it sets time to last update plus the given duration.

[github-actions]

Example alien_cake_addict failed to run, please try running it locally and check the result.

[github-actions]

Example alien_cake_addict failed to run, please try running it locally and check the result.

[github-actions]

Example scene failed to run, please try running it locally and check the result.

[github-actions]

Example load_gltf failed to run, please try running it locally and check the result.

[mockersf]

Example screenshots can be found in the artefacts section of https://github.com/bevyengine/bevy/actions/runs/4800302635

[JMS55]

Would be awesome to use these CI-generated images in the examples page of the website :)

[mockersf]

That will be the plan after the next release (if this gets merged)!

[alice-i-cecile]

I really like this idea in theory, and the API is good. I'm curious to see how this ends up being used and how new screenshots can be blessed, but I'm content with this as a first step.

[TheRawMeatball]

There's definitely more work to be done, but this seems like a very good first step to me.