[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/include/app.h

@@ -46,6 +46,7 @@ bool app_isFormatValid(void);
 void app_updateCursorPos(double x, double y);
 void app_updateWindowPos(int x, int y);
 void app_handleResizeEvent(int w, int h, double scale, const struct Border border);
+void app_invalidateWindow(void);
 
 void app_handleMouseRelative(double normx, double normy,
     double rawx, double rawy);