avoid ping-ponging focus updates with multiple windows

The bug here was that each paint call in a window would update
the focus state of its panes to reflect the one that had focus.

However, it didn't account for the actual window focus; it would
just assume that it was focused.

The result was that the perceived focus would alternate between each of
the windows in the wezterm process, and if you were running an
application that had enabled focus tracking, those events could cause a
repaint and drive up the CPU utilization.

This commit addresses that by gating the focus update to only occur
when we have the focus, and for extra safety, avoid generating focus
events at the terminal layer if the new state matches the current state.

refs: https://github.com/wez/wezterm/issues/1838

term/src/terminalstate/mod.rs

@@ -719,6 +719,9 @@ impl TerminalState {
 
     /// Advise the terminal about a change in its focus state
     pub fn focus_changed(&mut self, focused: bool) {
+        if focused == self.focused {
+            return;
+        }
         if !focused {
             // notify app of release of buttons
             let buttons = self.current_mouse_buttons.clone();

wezterm-gui/src/termwindow/render.rs

@@ -1614,15 +1614,18 @@ impl super::TermWindow {
 
         let panes = self.get_panes_to_render();
         let num_panes = panes.len();
+        let focused = self.focused.is_some();
 
         for pos in panes {
             if pos.is_active {
                 self.update_text_cursor(&pos.pane);
+                if focused {
                     pos.pane.advise_focus();
                     mux::Mux::get()
                         .expect("called on mux thread")
                         .record_focus_for_current_identity(pos.pane.pane_id());
                 }
+            }
             self.paint_pane_opengl(&pos, num_panes)?;
         }