Raise an error when performing a negative crop

[radarhere]

Resolves #5957

At the moment, mixing up the order of the box values for crop(), so that a negative crop is performed, doesn't raise an error.

from PIL import Image
im = Image.new("RGB", (10, 10))
im.crop((10, 10, 0, 0))

gives a (0, 0) image.

In the issue, this was done unintentionally by a user, and because no error was raised, they didn't realise their mistake.

This PR raises an error is the right of a crop region is less than the left, or if the bottom of the region is less than the top.

I am presuming that it is uncommon for users to want a negative crop, and more common for users to mix up the order of the values.

[alexwlchan]

Looks good to me, thanks!

[hugovk]

Thanks!