Progress estimate for a `VNVideoProcessor` operation

Would might be a good approach to estimating a VNVideoProcessor operation? I'd like to show a progress bar that's useful enough like one based the progress Apple vends for the photo picker or exports. This would make a world of difference compared to a UIActivityIndicatorView, but I'm not sure how to approach handrolling this (or if that would even be a good idea).

I filed an API enhancement request for this, FB9888210.