Previews a brush stroke using this tool on the specified layer (returning the final brushstroke so it can be added to the canvas)
(button_down: Button, pointer_id: PointerId, input: &mut InputStream<FocusEvent>, context: &SceneContext, data: Arc<Mutex<BrushToolState>>, (namespace, layer): (NamespaceId, LayerId))
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no test coverage detected