refactor engine and layout into input and output

output/src/collection.rs

@@ -0,0 +1,23 @@
+//! Groupings of elements.
+use crate::*;
+
+/// A function or closure that emits renderables.
+pub trait Collector<E: Engine>: Send + Sync + Fn(&mut dyn FnMut(&dyn Render<E>)) {}
+
+/// Any function or closure that emits renderables for the given engine matches [CollectCallback].
+impl<E, F> Collector<E> for F
+where E: Engine, F: Send + Sync + Fn(&mut dyn FnMut(&dyn Render<E>)) {}
+
+pub trait Render<E: Engine> {
+    fn area (&self, to: E::Area) -> E::Area;
+    fn render (&self, to: &mut E::Output);
+}
+
+impl<E: Engine, C: Content<E>> Render<E> for C {
+    fn area (&self, to: E::Area) -> E::Area {
+        Content::area(self, to)
+    }
+    fn render (&self, to: &mut E::Output) {
+        Content::render(self, to)
+    }
+}