Fade out the edges of ScrollAreas (#8018)

## Before:
<img width="381" height="307" alt="image"
src="https://github.com/user-attachments/assets/0528ae2a-44bf-4d9e-89a4-c3f4ab438eb2"
/>

It is very hard here to realize this is a scrollable area

## After
<img width="383" height="310" alt="image"
src="https://github.com/user-attachments/assets/9e0ee6de-8b96-4e5c-a505-f57977010990"
/>

The fade at the bottom tells the user they should try scrolling.

You can turn if off with `style.spacing.scroll.fade.enabled`

crates/epaint/src/lib.rs

@@ -27,6 +27,7 @@ mod brush;
 pub mod color;
 mod corner_radius;
 mod corner_radius_f32;
+mod direction;
 pub mod image;
 mod margin;
 mod margin_f32;
@@ -50,6 +51,7 @@ pub use self::{
     color::ColorMode,
     corner_radius::CornerRadius,
     corner_radius_f32::CornerRadiusF32,
+    direction::Direction,
     image::{AlphaFromCoverage, ColorImage, ImageData, ImageDelta},
     margin::Margin,
     margin_f32::*,