This prevents high memory usage by non-proxied frames when doing mask parenting. Description from code: Originally was needed to support image sequences with different image dimensions, which might be useful for such things as reconstruction of unordered image sequence, or painting/rotoscoping of non-equal-sized images, but this ended up in unneeded cache lookups and even unwanted non-proxied files loading when doing mask parenting, so let's disable this for now and assume image sequence consists of images with equal sizes