Save null value for crop if user didn't select a specific region
Rename variable to show difference to other variable
Check if cropX or cropY is 0 because it results in image-bug

If drawing a rectangle at topmost or leftmost position with 0 width it will
have at least 1px which is wrong. So no rectangle should be drawn if cropX
or cropY is zero.
Only ignore part of cropData if set to null, not if zero