Use iTerm2 "synchronised update" sequences around redraws.

In terminals that support it, this sequence causes the terminal to not redraw
*its* output until the application has finished, reducing redraw flickering.

The sequence is defined in:

    https://gitlab.com/gnachman/iterm2/-/wikis/synchronized-updates-spec

...and is apparently also supported by kitty and libVTE.

Fixes #3482.

src/terminal_ui.cc

@@ -226,6 +226,9 @@ void TerminalUI::Screen::output(bool force)
     if (lines.empty())
         return;
 
+    // iTerm2 "begin synchronised update" sequence
+    printf("\033P=1s\033\\");
+
     struct Change { int keep; int add; int del; };
     Vector<Change> changes{Change{}};
     auto new_hashes = lines | transform([](auto& line) { return hash_value(line.atoms); }) | gather<Vector>();
@@ -293,6 +296,9 @@ void TerminalUI::Screen::output(bool force)
             }
         }
     }
+
+    // iTerm2 "endsynchronised update" sequence
+    printf("\033P=2s\033\\");
 }
 
 constexpr int TerminalUI::default_shift_function_key;