Feature: add pixel_threshold for tests

[bircni]

I thought about this - so we have two options here:

1. adding it to SnapshotOptions
2. adding it to every function which I do not like as this would be a huge breaking change

Summary

This pull request introduces a new feature to the SnapshotOptions struct in the egui_kittest crate, allowing users to specify a permissible percentage of pixel differences (diff_percentage) before a snapshot comparison is considered a failure. This feature provides more flexibility in handling minor visual discrepancies during snapshot testing.

Additions to SnapshotOptions:

• Added a new field diff_percentage of type Option<f64> to the SnapshotOptions struct. This field allows users to define a tolerance for pixel differences, with a default value of None (interpreted as 0% tolerance).
• Updated the Default implementation of SnapshotOptions to initialize diff_percentage to None.

Integration into snapshot comparison logic:

• Updated the try_image_snapshot_options function to handle the new diff_percentage field. If a diff_percentage is specified, the function calculates the percentage of differing pixels and allows the snapshot to pass if the difference is within the specified tolerance. [1] [2]

• Closes Add pixel_threshold and improve threshold docs #5683

• I have followed the instructions in the PR template

[bircni]

To discuss:
Should this be an Option or just have a default value like 0.01 ?

I would be fine with both but I think per default it should be 0

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7092-patch5683
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

I'd prefer if this was a threshold of broken pixels instead of percentage, which would be more convenient since it's also what the error prints on test failure.
e.g. in

'easymarkeditor' Image did not match snapshot. Diff: 420, "/Users/runner/work/egui/egui/crates/egui_demo_app/tests/snapshots/easymarkeditor.diff.png". Run `UPDATE_SNAPSHOTS=1 cargo test --all-features` to update the snapshots.

420 means 420 pixels were above the threshold.
If we had a pixel_threshold option you could just use the 420 instead of having to calculate or guess the percentage.

Also, it'd be nice to have a helper to add a per-platform threshold. It could just be a struct with {windows: f32, linux: f32, ...} and impl Into so you could use it directly on the threshold param.

[bircni]

@lucasmerlin done :-)

[lucasmerlin]

Awesome!

[bircni]

@lucasmerlin fixed

[bircni]

Rebased to keep it ready to merge