GWindow: Don't send InvalidateRect message with 0 rects to WindowServer.

This could happen if GWindow::hide() was called while a window had pending
repaint rects. The deferred_invoke() lambda in GWindow::update() would
eventually run, but by then, hide() had cleared all pending rects.

LibGUI/GWindow.cpp

@@ -317,17 +317,19 @@ void GWindow::update(const Rect& a_rect)
 
     if (m_pending_paint_event_rects.is_empty()) {
         deferred_invoke([this] (auto&) {
+            auto rects = move(m_pending_paint_event_rects);
+            if (rects.is_empty())
+                return;
             WSAPI_ClientMessage request;
             request.type = WSAPI_ClientMessage::Type::InvalidateRect;
             request.window_id = m_window_id;
-            for (int i = 0; i < min(WSAPI_ClientMessage::max_inline_rect_count, m_pending_paint_event_rects.size()); ++i)
-                request.rects[i] = m_pending_paint_event_rects[i];
+            for (int i = 0; i < min(WSAPI_ClientMessage::max_inline_rect_count, rects.size()); ++i)
+                request.rects[i] = rects[i];
             ByteBuffer extra_data;
-            if (m_pending_paint_event_rects.size() > WSAPI_ClientMessage::max_inline_rect_count)
-                extra_data = ByteBuffer::wrap(&m_pending_paint_event_rects[WSAPI_ClientMessage::max_inline_rect_count], (m_pending_paint_event_rects.size() - WSAPI_ClientMessage::max_inline_rect_count) * sizeof(WSAPI_Rect));
-            request.rect_count = m_pending_paint_event_rects.size();
+            if (rects.size() > WSAPI_ClientMessage::max_inline_rect_count)
+                extra_data = ByteBuffer::wrap(&rects[WSAPI_ClientMessage::max_inline_rect_count], (rects.size() - WSAPI_ClientMessage::max_inline_rect_count) * sizeof(WSAPI_Rect));
+            request.rect_count = rects.size();
             GEventLoop::current().post_message_to_server(request, extra_data);
-            m_pending_paint_event_rects.clear_with_capacity();
         });
     }
     m_pending_paint_event_rects.append(a_rect);