background fit bugfix, typecast fix, GwyImage data setter fix

[ngergihun]

No description provided.

[markotoplak]

I merged, but implicit cast to integers within __init__ is not nice. Instead, it should crash (=raise an exception) if it is not integer.

I like that libraries do not do anything unexpected and that they complain loudly whenever something is unusual. Oh the other hand, GUI applications should just do as many necessary adaption as needed to get some output BUT they should still somehow warn you that something strange was done.

[ngergihun]

Thanks Marko for the suggestions. What is the standard way to make the methods more compatible with GUI usage? Just returning some default values and raising an error in case of calculation goes wrong?