Add Galley::intrinsic_size and use it in AtomLayout (#7146)

- part of https://github.com/emilk/egui/issues/5762
- also allows me to simplify sizing logic in egui_flex

tests/egui_tests/tests/test_atoms.rs

@@ -69,3 +69,35 @@ fn single_test(name: &str, mut f: impl FnMut(&mut Ui)) -> SnapshotResult {
 
     harness.try_snapshot(name)
 }
+
+#[test]
+fn test_intrinsic_size() {
+    let mut intrinsic_size = None;
+    for wrapping in [
+        TextWrapMode::Extend,
+        TextWrapMode::Wrap,
+        TextWrapMode::Truncate,
+    ] {
+        _ = HarnessBuilder::default()
+            .with_size(Vec2::new(100.0, 100.0))
+            .build_ui(|ui| {
+                ui.style_mut().wrap_mode = Some(wrapping);
+                let response = ui.add(Button::new(
+                    "Hello world this is a long text that should be wrapped.",
+                ));
+                if let Some(current_intrinsic_size) = intrinsic_size {
+                    assert_eq!(
+                        Some(current_intrinsic_size),
+                        response.intrinsic_size,
+                        "For wrapping: {wrapping:?}"
+                    );
+                }
+                assert!(
+                    response.intrinsic_size.is_some(),
+                    "intrinsic_size should be set for `Button`"
+                );
+                intrinsic_size = response.intrinsic_size;
+            });
+    }
+    assert_eq!(intrinsic_size.unwrap().round(), Vec2::new(305.0, 18.0));
+}