Run a ui without a window, and test one

`Tasks` held an `Arc<Window>` only to call `request_redraw` when a task
finished, which made the task queue -- and so `DefaultRsc` -- impossible
to build without a window. It now takes an `Arc<dyn WakeTaskQueue>`, and
`Window` implements it.

Waking moves from "the task ended" to "an update was sent", which is when
there is something for the host to apply: a task that keeps running after
sending one no longer holds its update until it finishes, and a task that
sends none no longer asks for a frame it does not need.

`iris::harness` is what that buys. `UiRenderState` already does layout,
hit testing and primitive building with no surface, so a test can build a
tree, run frames, move a pointer and read back where widgets landed.
`tests/harness.rs` does each of those; none of them could be written
before, since the only entry point to layout was a window.

It does not draw. A claim about pixels still needs a real surface.

core/src/ui/render_state.rs

@@ -34,6 +34,10 @@ impl UiRenderState {
         self.resized = true;
     }
 
+    pub fn output_size(&self) -> Vec2 {
+        self.output_size
+    }
+
     pub fn update<'a>(&mut self, root: impl Into<Option<&'a StrongWidget>>, rsc: &mut dyn UiRsc) {
         // safety mechanism for memory leaks; might wanna return a result instead so user can
         // decide whether to panic or not