PixelPaint: Update editor after drawing shapes to clear outside

Previously, we didn't ask the editor to update after drawing a
Rectangle/Line. This meant that if any part of your shape went
outside the bounds of the image, that part would not be cleared out
until the next update of the editor.

Userland/Applications/PixelPaint/RectangleTool.cpp

@@ -80,6 +80,7 @@ void RectangleTool::on_mouseup(Layer* layer, MouseEvent& event)
         draw_using(painter, m_rectangle_start_position, m_rectangle_end_position, m_thickness);
         m_drawing_button = GUI::MouseButton::None;
         layer->did_modify_bitmap();
+        m_editor->update();
         m_editor->did_complete_action();
     }
 }