Fix rounding in steps easing function

[rparrett]

Objective

While working on #17742, I noticed that the Steps easing function looked a bit suspicious.

Comparing to the options available in css:

It is "off the charts," so probably not what users are expecting.

Solution

Use floor when rounding to match the default behavior (jump-end, top right) in css.

Testing

I had to modify an existing test that was testing against the old behavior. This function and test were introduced in #14788 and I didn't see any discussion about the rounding there.

cargo run --example easing_functions

Migration Guide

EaseFunction::Steps now behaves like css's default, "jump-end." If you were relying on the old behavior, we plan on providing it. See #17744.

[rparrett]

cc @RobWalt

[RobWalt]

LGTM, thanks!

I think it would be good to support all of the CSS modi of steps via configuration options somewhere. Can you open an issue and link to this PR please? I'll tackle it later on when I'm back home (on mobile atm)

[Carter0]

Can you take a screenshot of what it looks like with your changes?

It does look odd on close inspection. It starts like halfway through a step I think. And then I swear the vertical lines are slightly bent. Maybe I’m seeing things tho

[rparrett]

It starts like halfway through a step I think.

That's the appearance, but I would say "it advances half-way through the step."

And then I swear the vertical lines are slightly bent. Maybe I’m seeing things tho

That's to be expected based on the sampling that's happening in the example.

Can you take a screenshot of what it looks like with your changes

Sure

[rparrett]

Looking into the failing test. It succeeds locally.

edit: Oh, oops. Just failed to push the new test code.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.