RenderData.GetContentBounds should cache bounds when possible

I am drawing many lines into a DrawingVisual and based on profiling it is taking longer to compute the bounds each time Visual.PrecomputeContent is called than it is to render it! The lines are all drawn with a frozen pen so it should be possible to know that the DrawingVisual cannot change and cache the bounding box. I would even be willing to have a OverrideBounds property on DrawingVisual that I can set when I know the bounding box won't change.