winit-software renderer leaves artifacts when text moves

### Bug Description See code below. It's from the docs and is a simple rectangle that follows the cursor and shows the x,y coordinates as text. On the default winit-femtovg renderer, it works as expected. On winit-software, the text does not always get cleared when the cursor/rectangle moves around on the screen. To reproduce, set environment variable `SLINT_BACKEND=winit-software` and move the cursor around for a bit. Screenshot below. <img width="530" height="558" alt="Image" src="https://github.com/user-attachments/assets/32ca1f0f-df0e-4c47-aa63-94269be2f33a" /> ### Reproducible Code (if applicable) ```slint export component MainWindow inherits Window { width: 400px; height: 400px; ta := TouchArea{} rect := Rectangle { x: ta.mouse-x; y: ta.mouse-y; width: 60px; height: 60px; background: ta.pressed ? orange : skyblue; Text { x: 5px; y: 5px; text: "x: " + rect.x / 1px; } Text { x: 5px; y: 20px; text: "y: " + rect.y / 1px; } } } ``` ### Environment Details - Slint Version: 1.16.1 - Platform/OS: Arch Linux KDE 6.7.0 Wayland - Programming Language: Rust - Backend/Renderer: winit-software ### Product Impact Still learning the ropes for a project I have in mind. I imagine most users will be using a GPU accelerated backend so not a dealbreaker.