Split text measurements cache from rendering cache

This speeds up layouting in the most common scenario considerably!
🎉

native/src/renderer.rs

@@ -26,6 +26,15 @@ mod windowed;
 pub use debugger::Debugger;
 pub use windowed::{Target, Windowed};
 
-pub trait Renderer {
+use crate::{layout, Element};
+
+pub trait Renderer: Sized {
     type Output;
+
+    fn layout<'a, Message>(
+        &mut self,
+        element: &Element<'a, Message, Self>,
+    ) -> layout::Node {
+        element.layout(self, &layout::Limits::NONE)
+    }
 }

native/src/user_interface.rs

@@ -97,7 +97,7 @@ where
     pub fn build<E: Into<Element<'a, Message, Renderer>>>(
         root: E,
         cache: Cache,
-        renderer: &Renderer,
+        renderer: &mut Renderer,
     ) -> Self {
         let root = root.into();
 
@@ -110,7 +110,7 @@ where
             cache.layout
         } else {
             let layout_start = std::time::Instant::now();
-            let layout = root.layout(renderer, &layout::Limits::NONE);
+            let layout = renderer.layout(&root);
             dbg!(std::time::Instant::now() - layout_start);
 
             layout