fix unintuitive default mame->retropad mapping

[markwkidd]

@shakkalakka has prepared extensive notes on the issue of the unintuitive/difficult to use default input mapping of this core: https://forums.libretro.com/t/default-input-mapping-for-mame2014-and-newer-is-wrong/11281

I'm submitting this PR on behalf of shakkalakka because they are concerned about their language barrier.

Many of the popular games already have coded independent defaults farther down in the source. In other words, this default does not change how every game is handled.

One way to evaluate the change would be to try to come up with any prominent examples where this would not be more intuitive.

[sergiobenrocha2]

Ugh, we shouldn't change that...

Right now, MAME layout == FB Alpha layout (arcade scheme) [well, mostly]

If you change the default layout, then MAME layout == FB Alpha layout (gamepad scheme) ?

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Example with Beat 'em ups:

A: attack
B: jump

With this PR, you'll get:

Y: attack
B: jump

... but then, it'll not match with some games like Final Fight (CPS-1), which has A: attack and B: jump. You can't just change the default layout without changing other layouts (CPS-1/2, Neo Geo and others), it'll break consistency between games.

[sergiobenrocha2]

MAME cores should have a switch like FB Alpha "Control Scheme (gamepad/arcade)". This PR is a no-no

[shakkalakka]

hello people,
to me it seems most games(*) are not mapped "FBA-like" for now... (i have compared to both fba-modes gamepad and arcade and against the default standalone mame-mapping layout).
i wonder it was the same layout for the mame-core, too!

*except the "hand-corrected" ones (wich are mostly fighters only).

for explaining:
on my Standalone-Mame i just hooked up the default Keyboard Mapping from Mame to my Input-Devices. in this way i can use the default layout for any Controller and this one basicly matches the fba-layout.
with this "default" Mapping all i needed is correcting the fighters and few others (very similar to your approach)
BUT the very-most part of the mame gamelibary plays very fine out of the box with most authentic controll-scheme. just like FBA in many cases.

EDIT: have tried Altered Beast and DD1+2 and its playing very nicely with the new mapping. to sad that the most part of games is still messed up controll-layoutwise (Doulbe Dragon 3 for obvious example). But for now I Thank you very very much for working on it!!!!