Finally I came to a first working version of the 2D GUI based on the Cairo libraries. The entire GUI architecture is based on 2D Engine components like the graphics and the physics engines. The graphics engine makes use of graphics context that in this implementation is based on Cairo, but it can be specialized with any library.
As you can see in the video, I reused some old skin from WindowsXP, but the skin is totally programmable and it will be changed in the future. For now there are only simple widgets like: form windows, buttons, options and check boxes. The next step is to implement other composed widgets like scroll bars, text boxes, tabs, lists, treeviews and so on. This GUI can be used for video games or to produce professional applications. The GUI is designed to run on full screen or using the widgets of the operating system. The full screen variant can be specialized to work with GPU libraries, like Direct3D or Vulkan. As a modern feature, a transform matrix can be applied to every widget, so they can be translated, rotated, scaled or skewed with matrix operations. The interface can be designed with an external editor and not with code embedded inside the application. The only code required on the application side is the one used to manage the widget events.