perf counter for render

tui/src/tui_engine.rs

@@ -5,6 +5,7 @@ pub struct Tui {
     pub backend: CrosstermBackend<Stdout>,
     pub buffer:  Buffer,
     pub area:    [u16;4],
+    pub perf:    PerfModel,
 }
 
 impl Tui {
@@ -17,6 +18,7 @@ impl Tui {
             buffer:  Buffer::empty(Rect { x: 0, y: 0, width, height }),
             area:    [0, 0, width, height],
             backend,
+            perf: Default::default(),
         })))
     }
     /// True if done