script: Ensure that leaving the WebView sets the cursor back to the default cursor (#38759)

This changes makes a variety of changes to ensure that the cursor is set
back to the default cursor when it leaves the `WebView`:

1. Display list updates can come after a mouse leaves the `WebView`, so
   when refreshing the cursor after the update, base the updated cursor
   on the last hovered location in the `DocumentEventHandler`, rather
   than the compositor. This allows us to catch when the last hovered
   position is `None` (ie the cursor has left the `WebView`).
2. When handling `MouseLeftViewport` events for the cursor leaving the
   entire WebView, properly set the
   MouseLeftViewport::focus_moving_to_another_iframe` on the input event
   passed to the script thread.
3. When moving out of the `WebView` entirely, explicitly ask the
   embedder to set the cursor back to the default.

Testing: This change adds a unit test verifying this behavior.
Fixes: #38710.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/compositing/compositor.rs

@@ -1601,7 +1601,6 @@ impl IOCompositor {
                 .constellation_sender
                 .send(EmbedderToConstellationMessage::RefreshCursor(
                     hit_test_result.pipeline_id,
-                    hit_test_result.point_in_viewport,
                 ))
         {
             warn!("Sending event to constellation failed ({:?}).", error);