macosx: Fix dark/light switch for VLCImageView border
Also has a bit of a clean-up of the VLCImageView internals
After fix:
Screen_Recording_2023-07-12_at_16.38.47
Before fix:
Screen_Recording_2023-07-12_at_16.57.39
Signed-off-by: Claudio Cambra developer@claudiocambra.com