Partial rerender via raw window handle

[floers]

I want to create a slint app where I can control which parts are rerendered (damaged). This is because it is a pen drawing app and I want e-ink devices to render only the pen strokes. I am aware this is not supported by slint directly atm (#3190) but shouldn't it be possible to use the raw window handle?

I render the pen strokes via opengl (femtovg::renderer::opengl::OpenGL) via window.set_rendering_notifier. Now instead of rerendering this on rerender request I thought I can do this on input event and then use wayland directly

// code that 
surface.damage_buffer(x,y,w,h);
surface.commit();

This doesn't work though and I think it is because slint did not change the underlying buffer unless request_redraw is called? But this also makes the compositor rerender the whole window. Could I get the surface buffer somehow and write my pen strokes directly inside? I am blocked by this and maybe on the wrong track. Is there any way I could accomplish my goal? If not: I would be happy to contribute to slint to make this possible but I would need some hints where and how this should be added...