script: Remove note_rendering_opportunity and rendering_opportunity (#34575)

A rendering opportunity is now unconditionally triggered by handling IPC
messages in the `ScriptThread`, unless animations are running in which
case it's driven by the compositor. We can now remove calls to
`note_rendering_opportunity` and `rendering_opportunity`.

There is one tricky case, which is when a promise completion during a
microtask checkpoint dirties the page again. In this case we need to
trigger a new rendering opportunity, unless animations are running.  In
a followup change, when not driven by the compositor, rendering
opportunities will be driven by a timed task, meaning we can remove this
workaround.

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

components/script/task_source/rendering.rs

@@ -43,3 +43,19 @@ impl TaskSource for RenderingTaskSource {
         self.0.send(msg_task).map_err(|_| ())
     }
 }
+
+impl RenderingTaskSource {
+    /// This queues a task that will not be cancelled when its associated
+    /// global scope gets destroyed.
+    pub fn queue_unconditionally<T>(&self, task: T) -> Result<(), ()>
+    where
+        T: TaskOnce + 'static,
+    {
+        self.0.send(CommonScriptMsg::Task(
+            ScriptThreadEventCategory::NetworkEvent,
+            Box::new(task),
+            Some(self.1),
+            RenderingTaskSource::NAME,
+        ))
+    }
+}