When ignoring an update, request one for later

If we were not ready to paint the screen, this does not mean that we
should give up on repainting, just potpone it.

I thought that it was bad to call update() in the paint event, but I
cannot find reference to this anymore and everything seems to work as
intended.

src/frontends/qt4/GuiWorkArea.cpp

@@ -1271,6 +1271,8 @@ void GuiWorkArea::paintEvent(QPaintEvent * ev)
 		// backing store has to be copied to screen (this is a no-op
 		// except on macOS).
 		d->updateScreen(ev->rect());
+		// Ignore this paint event, but request a new one for later.
+		viewport()->update();
 		ev->accept();
 		return;
 	}