refactor engine and layout into input and output

output/src/size.rs

@@ -0,0 +1,40 @@
+use crate::*;
+use std::fmt::Debug;
+
+pub trait Size<N: Coordinate>: From<[N;2]> + Debug + Copy {
+    fn x (&self) -> N;
+    fn y (&self) -> N;
+    #[inline] fn w (&self) -> N { self.x() }
+    #[inline] fn h (&self) -> N { self.y() }
+    #[inline] fn wh (&self) -> [N;2] { [self.x(), self.y()] }
+    #[inline] fn clip_w (&self, w: N) -> [N;2] { [self.w().min(w), self.h()] }
+    #[inline] fn clip_h (&self, h: N) -> [N;2] { [self.w(), self.h().min(h)] }
+    #[inline] fn expect_min (&self, w: N, h: N) -> Usually<&Self> {
+        if self.w() < w || self.h() < h {
+            Err(format!("min {w}x{h}").into())
+        } else {
+            Ok(self)
+        }
+    }
+    #[inline] fn zero () -> [N;2] {
+        [N::zero(), N::zero()]
+    }
+    #[inline] fn to_area_pos (&self) -> [N;4] {
+        let [x, y] = self.wh();
+        [x, y, 0.into(), 0.into()]
+    }
+    #[inline] fn to_area_size (&self) -> [N;4] {
+        let [w, h] = self.wh();
+        [0.into(), 0.into(), w, h]
+    }
+}
+
+impl<N: Coordinate> Size<N> for (N, N) {
+    #[inline] fn x (&self) -> N { self.0 }
+    #[inline] fn y (&self) -> N { self.1 }
+}
+
+impl<N: Coordinate> Size<N> for [N;2] {
+    #[inline] fn x (&self) -> N { self[0] }
+    #[inline] fn y (&self) -> N { self[1] }
+}