[client] all: make it possible to signal full window invalidation

Now that we are drawing with damage rects, when the window is hidden and
then exposed the window may not get fully redrawn. This provides
`app_invalidateWindow` for the display server backend to call when the
screen needs a full redraw.

client/src/main.h

@@ -30,6 +30,7 @@
 #include "common/ivshmem.h"
 #include "common/locking.h"
 #include "common/ringbuffer.h"
+#include "common/event.h"
 
 #include "spice/spice.h"
 #include <lgmp/client.h>
@@ -103,6 +104,8 @@ struct AppState
   PLGMPClientQueue     pointerQueue;
 
   LGThread            * frameThread;
+  LGEvent             * frameEvent;
+  atomic_bool           invalidateWindow;
   bool                  formatValid;
   atomic_uint_least64_t frameTime;
   uint64_t              lastFrameTime;