Support for negative minimumLineSpacing

[J-Rojas]

The standard UICollectionViewFlowLayout supports the use of negative minimumLineSpacing to allow overlapping rows.

KTCenterFlowLayout uses some internal rectangle intersection logic that prevents detection of overlapping rows and considers all items to be on in the same row during its layout calculation, resulting in an incorrect layout.

I'm going to attempt a fix and will submit a PR.