fix/workaround opencv assertion failure
fix/workaround opencv assertion failure

Add a new size flag, eg. `-s 1280x960` to ignore images not matching the
specified size. Not sure where the improperly sized images are coming from
(maybe truncated writes to disk?) but this allows me to just reject them
and keep OpenCV from exploding.

thomasjacquin/allsky,thomasjacquin/allsky,thomasjacquin/allsky