GB Video: Fix underflow
GB Video: Fix underflow

MerryMage/mgba,mgba-emu/mgba,mgba-emu/mgba,sergiobenrocha2/mgba,Iniquitatis/mgba,libretro/mgba,MerryMage/mgba,Iniquitatis/mgba,iracigt/mgba,jeremyherbert/mgba,fr500/mgba,libretro/mgba,Touched/mgba,libretro/mgba,fr500/mgba,sergiobenrocha2/mgba,Anty-Lemon/mgba,iracigt/mgba,iracigt/mgba,Anty-Lemon/mgba,mgba-emu/mgba,sergiobenrocha2/mgba,jeremyherbert/mgba,Touched/mgba,Anty-Lemon/mgba,Anty-Lemon/mgba,Iniquitatis/mgba,iracigt/mgba,Iniquitatis/mgba,jeremyherbert/mgba,libretro/mgba,mgba-emu/mgba,jeremyherbert/mgba,libretro/mgba,fr500/mgba,sergiobenrocha2/mgba,fr500/mgba,MerryMage/mgba,sergiobenrocha2/mgba,Touched/mgba