libservo|compositor: Have scroll offset directionality match that of WebRender and the web (#37752)

Previously, our Servo-specific spatial tree scroll offsets were opposite
to
that of WebRender and also the web platform. This is due to the fact,
likely, that `winit` wheel directionality is also flipped. This change
has both the Servo spatial tree and the API take offsets that are
consistent with the web.

Any possible changes to the meaning of wheel directionality will be
handled in a followup change.

This is a breaking change to the Servo API.

Testing: This change updates unit tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout/layout_impl.rs

@@ -476,7 +476,6 @@ impl Layout for LayoutThread {
             .borrow_mut()
             .as_mut()
             .and_then(|tree| tree.compositor_info.scroll_tree.scroll_offset(id))
-            .map(|scroll_offset| -scroll_offset)
     }
 }
 
@@ -1083,7 +1082,7 @@ impl LayoutThread {
             .scroll_tree
             .set_scroll_offset_for_node_with_external_scroll_id(
                 external_scroll_id,
-                -offset,
+                offset,
                 ScrollType::Script,
             )
         {