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/WindowServer.ipc

@@ -144,4 +144,6 @@ endpoint WindowServer
     get_double_click_speed() => (int speed)
 
     get_desktop_display_scale(u32 screen_index) => (int desktop_display_scale)
+
+    set_flash_flush(bool enabled) =|
 }