refactor engine and layout into input and output

output/src/when.rs

@@ -0,0 +1,23 @@
+use crate::*;
+
+/// Show an item only when a condition is true.
+pub struct When<A>(pub bool, pub A);
+
+impl<E: Output, A: Render<E>> Content<E> for When<A> {
+    fn layout (&self, to: E::Area) -> E::Area {
+        let Self(cond, item) = self;
+        let mut area = E::Area::zero();
+        if *cond {
+            let item_area = item.layout(to);
+            area[0] = item_area.x();
+            area[1] = item_area.y();
+            area[2] = item_area.w();
+            area[3] = item_area.h();
+        }
+        area.into()
+    }
+    fn render (&self, to: &mut E) {
+        let Self(cond, item) = self;
+        if *cond { item.render(to) }
+    }
+}