A basic Slint app that uses a custom backend implemented with SDL3. The example demonstrates how to create a Wayland subsurface and use it as a target for Vulkan rendering that operates independently from the Slint renderer. When the app runs, Slint displays its UI using the Skia renderer, while a Vulkan renderer draws a colored triangle to the subsurface. UI changes, such as resizing the window or adjusting the scale factor, automatically update the subsurface geometry so that Vulkan graphics remain properly positioned and scaled alongside the Slint interface. The result is a window where both Slint UI elements and Vulkan-rendered content coexist and update responsively.