Slap an InterruptDisabler on gui$invalidate_window().

This is obviously not a permanent solution but it works now to allow
the windowing system to withstand invalidation spam.

Kernel/ProcessGUI.cpp

@@ -113,6 +113,7 @@ int Process::gui$invalidate_window(int window_id)
     auto& window = *(*it).value;
     // FIXME: This should queue up a message that the window server process can read.
     //        Poking into its data structures is not good.
+    InterruptDisabler disabler;
     WindowManager::the().invalidate(window);
     return 0;
 }