A value that indicates that rendered content should be cached as a composited bitmap when possible.

https://docs.microsoft.com/en-us/dotnet/desktop/wpf/graphics-multimedia/how-to-improve-rendering-performance-by-caching-an-element
https://docs.microsoft.com/en-us/windows/winui/api/microsoft.ui.xaml.uielement.cachemode?view=winui-3.0

<Canvas Background="White" CacheMode="BitmapCache">
    <Ellipse Height="40" Width="40" Fill="Blue"/>
    <Ellipse Canvas.Left="21" Height="40" Width="40" Fill="Blue"/>
    <Ellipse Canvas.Top="13" Canvas.Left="10" Height="40" Width="40" Fill="Blue"/>
</Canvas>

We probably want to use an atlas for this. Right now we have one for offscreens and effects that is cleared per frame. The one we need here needs to persists between frames, so we need to expose a way to bind a render target without clearing its content in the RenderDevice abstraction.