Pocket Bomberman SGB borders non-working

[vaguerant]

Testing on 9d3b445, the Super Game Boy borders don't work on Pocket Bomberman when mGBA is emulating a Super Game Boy. ROMs tested (No-Intro naming):

• Pocket Bomberman (Europe) (SGB Enhanced).gb
• Pocket Bomberman (Japan) (SGB Enhanced).gb
• Pocket Bomberman (USA, Europe) (SGB Enhanced) (GB Compatible).gbc

The same issue occurs across all versions.

Screenshot comparison between BGB (left) and mGBA (right):

Otherwise, palettes appear to work as expected.