LibWeb: Don't attempt to create new bitmap for zero-size OOPWV

It's not possible to construct a Gfx::Bitmap with empty size. Let the
client know the new viewport rect and return before even attempting to
create new front and back bitmaps.

Also consider that we might have to paint the widget but not have a
front/back bitmap available (e.g. when only part of a scrollbar is
visible, and the inner rect is empty).

Libraries/LibWeb/OutOfProcessWebView.cpp

@@ -71,6 +71,10 @@ void OutOfProcessWebView::paint_event(GUI::PaintEvent& event)
 {
     GUI::ScrollableWidget::paint_event(event);
 
+    // If the available size is empty, we don't have a front or back bitmap to draw.
+    if (available_size().is_empty())
+        return;
+
     GUI::Painter painter(*this);
     painter.add_clip_rect(frame_inner_rect());
     painter.add_clip_rect(event.rect());
@@ -84,9 +88,14 @@ void OutOfProcessWebView::resize_event(GUI::ResizeEvent& event)
 {
     GUI::ScrollableWidget::resize_event(event);
 
+    client().post_message(Messages::WebContentServer::SetViewportRect(Gfx::IntRect({ horizontal_scrollbar().value(), vertical_scrollbar().value() }, available_size())));
+
     m_front_bitmap = nullptr;
     m_back_bitmap = nullptr;
 
+    if (available_size().is_empty())
+        return;
+
     // FIXME: Don't create a temporary bitmap just to convert it to one backed by a shared buffer.
     if (auto helper = Gfx::Bitmap::create(Gfx::BitmapFormat::RGB32, available_size())) {
         m_front_bitmap = helper->to_bitmap_backed_by_shared_buffer();
@@ -101,7 +110,6 @@ void OutOfProcessWebView::resize_event(GUI::ResizeEvent& event)
         m_back_bitmap->shared_buffer()->share_with(client().server_pid());
     }
 
-    client().post_message(Messages::WebContentServer::SetViewportRect(Gfx::IntRect({ horizontal_scrollbar().value(), vertical_scrollbar().value() }, available_size())));
     request_repaint();
 }