Control Separator widths, and less clipping in ScrollArea (#2665)

* Add Separator::grow and Separator::shrink

* Be more conservative with the clipping in ScrollArea:s

* Add test of the growing separator

* Improve test output

* Update changelog

* Add back a little bit more clipping

* Make the minimum scroll handle length a bit longer

* More clip rect tweaks

crates/egui_demo_lib/src/lib.rs

@@ -92,7 +92,8 @@ fn test_egui_zero_window_size() {
         let clipped_primitives = ctx.tessellate(full_output.shapes);
         assert!(
             clipped_primitives.is_empty(),
-            "There should be nothing to show"
+            "There should be nothing to show, has at least one primitive with clip_rect: {:?}",
+            clipped_primitives[0].clip_rect
         );
     }
 }