subsurface_widget: Handle viewport and set source rectangle

This is needed to crop subsurface when needed, instead of rendering
overlapping other things. And to offset while scrolling.

winit/src/platform_specific/wayland/event_loop/state.rs

@@ -1612,7 +1612,8 @@ impl SctkState {
             wp_fractional_scale,
 
             wl_buffer: None,
-            bounds: Some(bounds),
+            source: None,
+            destination: Some(bounds),
             transform:
                 cctk::wayland_client::protocol::wl_output::Transform::Normal,
             z: settings.z,