WindowServer+wsctl: Add a simple utility for toggling "flash flush"

You can now put the WindowServer into "flash flush" mode by doing:

$ wsctl -f 1

To disable it, somewhat obviously:

$ wsctl -f 0

Userland/Services/WindowServer/ClientConnection.cpp

@@ -1099,4 +1099,9 @@ void ClientConnection::set_window_modified(i32 window_id, bool modified)
     window.set_modified(modified);
 }
 
+void ClientConnection::set_flash_flush(bool enabled)
+{
+    Compositor::the().set_flash_flush(enabled);
+}
+
 }