Allow masking to only track part of the image with Warp Stabilizer

In Warp Stabilizer sometimes I want to only use part of the image for tracking - if a moving object is filling a substantial part of the image it can produce unwanted results.

It's possible to manually delete the unwanted tracking points but this is very tedious. It would be very useful to be able to draw rough masks and use only the trackers within (or outside) that mask.