Fix: ScrollArea layout jitter with floating bars and zoom levels

[rustbasic]

Fix: ScrollArea layout jitter with floating bars and zoom levels

• Closes Unstable available_size in CentralPanel #7937
• Closes fix: Prevent widget jittering in justified layouts by applying a small offset #7942

This PR improves the stability of ScrollArea by addressing two layout issues:

1. Discrete Layout for Floating Bars: Fixed content "shaking" in floating mode when a non-zero allocated_width is used. By using discrete visibility (show_bars) instead of the animated factor for space allocation, we ensure the layout remains stable during scrollbar animations.
2. Zoom-level Stability: Introduced a small epsilon (0.1) when checking if content exceeds the viewport. This prevents scrollbars from flickering on and off due to floating-point rounding errors at specific zoom factors (e.g., 1.01 or 0.95).

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7944-patch174
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