Remove surface argument of Compositor::screenshot

This argument was completely ignored by the wgpu renderer, and used only
for the `clip_mask` by the `tiny_skia` renderer. I believe creating a
new clip mask is correct.

This way it's possible to render offscreen without needing a surface.

winit/src/program.rs

@@ -1456,7 +1456,6 @@ fn run_action<P, C>(
                 if let Some(window) = window_manager.get_mut(id) {
                     let bytes = compositor.screenshot(
                         &mut window.renderer,
-                        &mut window.surface,
                         window.state.viewport(),
                         window.state.background_color(),
                         &debug.overlay(),