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.

wgpu/src/window/compositor.rs

@@ -370,7 +370,6 @@ impl graphics::Compositor for Compositor {
     fn screenshot<T: AsRef<str>>(
         &mut self,
         renderer: &mut Self::Renderer,
-        _surface: &mut Self::Surface,
         viewport: &Viewport,
         background_color: Color,
         overlay: &[T],