mGBA system bus: I/O registers, VRAM, OAM, GamePak SRAM cannot be written manually through system bus

[getCursorsExe]

At http://problemkaputt.de/gbatek-gba-memory-map.htm these mentioned memory locations are suppossed to be read/write, but in mGBA you cannot poke them manually, just as if these memory locations were completely read-only.

[YoshiRulz]

See also #3524.

I'm guessing this is unintentional; the system bus is implemented separately from the other domains.

[CasualPokePlayer]

It's an "issue" upstream.
IO/PALRAM/VRAM/OAM are unimplemented
https://github.com/mgba-emu/mgba/blob/dbffb46c4e7d2e7a2cbed7c3488cece4c2176d4c/src/gba/memory.c#L1362-L1373
SRAM is partially implemented (only works when it's actually SRAM and not Flash/EEPROM/etc)
https://github.com/mgba-emu/mgba/blob/dbffb46c4e7d2e7a2cbed7c3488cece4c2176d4c/src/gba/memory.c#L1388-L1396

[nattthebear]

Since GBAPatch16 and GBAPatch32 do implement those things, we could fix it in the memory domain by adding separate writeshort and writelong implementations. This would probably be good enough, as in real usage you'd have little reason to write only 8 bits at a time to most of these locations.