Fix refresh bug

Terminal output wouldn't reliably refresh the screen. Fix is to only
modify this flag when the terminal lock is held which suggests a larger
refactoring should happen.

src/main.rs

@@ -211,10 +211,9 @@ fn main() {
         // Wait for something to happen
         processor.process_events(&window);
 
-        signal_flag.set(false);
-
         // Maybe draw the terminal
         let terminal = terminal.lock_high();
+        signal_flag.set(false);
         if terminal.dirty {
             display.draw(terminal);
         }