Nice to have feature: Image masking

[jksemple]

As well as cropping I also need to apply a circular mask to my images so that only the central circle is tested for image movement or anomaly detection. The rest of the image will be set to an arbitrary configured colour (e.g. black or white) for display purposes. It would be nice if this could be configured within the movement (or anomaly) detection parts of the library.

[eloquentarduino]

Added to my to-do list.