Enable VisualBoyAdvance core for GB/GBC

[BaffleBlend]

VisualBoyAdvance also supports Game Boy and Game Boy Color.

• Enable VBA core for GB/GBC systems

Certain Game Boy Color games have what's known as "GBA Enhanced Mode"*, where if the cartridge is inserted into a Game Boy Advance rather than a Game Boy Color, bonus content would be accessible.

Games with this mode include but are not limited to:

• Shantae
• The Legend of Zelda: Oracle of Seasons
• The Legend of Zelda: Oracle of Ages
• Dragon Quest III
• Wendy: Every Witch Way

^{The mode has no official name; the term "GBA Enhanced" was coined in Shantae.}

NOTE: attempts to load Game Boy or Game Boy Color games in the VisualBoyAdvance core have failed. I find this odd because, as a standalone program, VisualBoyAdvance can load those games fine.

[JoeMatt]

I took a quick stab at this by just adding the GameBoy controller callbacks and adding gb/gbc to the list of support systems.

It loads a rom, but just draws a blank white screen after that.
Perhaps more setup is needed for GB/GBC support.

Will have to look at what other front ends are doing

https://github.com/libretro/vba-next/blob/master/libretro/libretro.cpp

https://github.com/visualboyadvance-m/visualboyadvance-m

https://github.com/libretro/vbam-libretro/blob/master/src/libretro/libretro.cpp

Should also probably add palette and border options to menu
https://docs.libretro.com/library/vba_m/