Reserve scene at the beginning of the paint pass

[PoignardAzur]

This is an extension of the pass spec work (see #512).

Vello should provide a SceneMetrics type, which could be returned from a Scene, and multiple SceneMetrics could be added together. The Scene should have a new Scene::reserve(metrics) constructor taking a SceneMetrics value.

We could then keep individual metrics for each widget, sum them during the paint pass, and store the sum in the root. At the beginning of each paint pass, we'd use metrics from the previous pass to reserve a complete scene.

See linebender/vello#667.

[DJMcNab]

It seems to me that it would be much simpler to just retain the resultant scene from the previous frame. That would tend to grow over time, so we should probably have a shrink_to_fit method on Scene.