The low priority scene builder is an additional thread that can be used to perform some work before forwarding the transaction to the "normal" scene builder thread.
This lets us choose to rasterize very expensive content blobs without blocking the scene builder allowing updates of the UI disply list.
Since all low priority scenes request get forwarded to the regular scene builder as well, any operation that affects both threads (such as flushes or shutting down) can simply be sent to the low priority thread.