macosx: Ensure all layout attributes used and modified from super class are copied in collection view flow layout

Fixes following runtime warning

2024-09-09 20:36:14.447 vlc-osx-static[60831:4843507] VLCLibraryCollectionViewFlowLayout has cached frame mismatch for index path <NSIndexPath: 0x91865cbfc1b2ae5a> {length = 2, path = 0 - 6} - cached value: {{20, 575.66666666666674}, {162.83333333333334, 209.83333333333334}}; expected value: {{20, 565.66666666666674}, {162.83333333333334, 209.83333333333334}}
2024-09-09 20:36:14.447 vlc-osx-static[60831:4843507] This is likely occurring because the flow layout subclass VLCLibraryCollectionViewFlowLayout is modifying attributes returned by NSCollectionViewFlowLayout without copying them

Signed-off-by: Claudio Cambra developer@claudiocambra.com