Improve modifier handling when scrolling

[emilk]

Problem

Letting go of the modifier key before the last momentum-scroll events arrive will cause the scroll direction to change. This problem can be seen by going to egui.rs and opening the "Scene" example. Hold down shift, start a momentum-scroll (on a Mac trackpad), then quickly let go of shift: you'll see the scroll direction change, which feels wrong.

Solution

Store the modifiers at the start of the event, thanks to the new phase info added in

• Add is_scrolling/is_smooth_scrolling util, checking for active scroll action. #7669

Note that this solution only works on native; not on web.

Other

• Break out wheel/scroll handling into own file
• Simplify it a lot by deciding late on wether an input is a scroll or a zoom
• Assume input is already smooth if there are phase events

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7678-emilkscroll-refactor
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff