refactor engine and layout into input and output

output/src/output.rs

@@ -0,0 +1,22 @@
+use crate::*;
+
+/// Render target
+pub trait Output: Send + Sync + Sized {
+    /// Unit of length
+    type Unit: Coordinate;
+    /// Rectangle without offset
+    type Size: Size<Self::Unit>;
+    /// Rectangle with offset
+    type Area: Area<Self::Unit>;
+    /// Current output area
+    fn area (&self) -> Self::Area;
+    /// Mutable pointer to area
+    fn area_mut (&mut self) -> &mut Self::Area;
+    /// Render widget in area
+    fn place (&mut self, area: Self::Area, content: &impl Render<Self>);
+    #[inline] fn x (&self) -> Self::Unit { self.area().x() }
+    #[inline] fn y (&self) -> Self::Unit { self.area().y() }
+    #[inline] fn w (&self) -> Self::Unit { self.area().w() }
+    #[inline] fn h (&self) -> Self::Unit { self.area().h() }
+    #[inline] fn wh (&self) -> Self::Size { self.area().wh().into() }
+}