crossterm: Render with synchronized output

This takes advantage of the synchronized output commands from crossterm
to tell the terminal emulator when to pause and resume drawing. This
should feel snappier and have fewer visual artifacts like tearing when
we update the screen with many changes (scrolling or changing theme for
example).

helix-tui/src/backend/crossterm.rs

@@ -146,6 +146,10 @@ where
             }
         }
     }
+
+    fn supports_synchronized_output(&self) -> bool {
+        self.features().synchronized_output_mode != SynchronizedOutputMode::NotSupported
+    }
 }
 
 impl<W> Write for CrosstermBackend<W>
@@ -268,6 +272,10 @@ where
     where
         I: Iterator<Item = (u16, u16, &'a Cell)>,
     {
+        if self.supports_synchronized_output() {
+            queue!(self.buffer, terminal::BeginSynchronizedUpdate)?;
+        }
+
         let mut fg = Color::Reset;
         let mut bg = Color::Reset;
         let mut underline_color = Color::Reset;
@@ -326,7 +334,13 @@ where
             SetForegroundColor(CColor::Reset),
             SetBackgroundColor(CColor::Reset),
             SetAttribute(CAttribute::Reset)
-        )
+        )?;
+
+        if self.supports_synchronized_output() {
+            execute!(self.buffer, terminal::EndSynchronizedUpdate)?;
+        }
+
+        Ok(())
     }
 
     fn hide_cursor(&mut self) -> io::Result<()> {