Filtering mutually exclusive inputs.

[g6672D]

Right now it's possible to input up+down or left+right. This is possible on the real system but only with a modified or broken controller, and some games respond badly to them, so they should probably be filtered out, at least by default. It's also useful, notable example: http://www.youtube.com/watch?v=JZ6pDAqN7gQ

It should be mentioned this also affects the bsnes core, but that was addressed in v090 and can probably be backported. It also affects QuickNES and bnes but not Nestopia, some NES games also hate these inputs. I don't know how it is for other systems or cores.

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[Oggom]

Metal Slug Advance (and possibly other GBA games as well) suffer from this issue too libretro/vba-next#14