[Gameboy Feature] force cgb on gb game

[benderscruffy]

No description provided.

[jsgroth]

This is a little harder than implementing the "force DMG mode" option was because DMG-on-CGB compatibility mode works differently from normal CGB mode. The hardware basically tries to mimic DMG behavior as closely as possible, except instead of displaying the 2-bit DMG colors from BGP/OBP0/OBP1, it uses them to index into CGB palette RAM. It expects that the CGB boot process pre-populated CGB BG palette 0 and CGB OBJ palettes 0/1 with useful colors.

This feature won't be very useful without also adding a way to initialize the CGB palettes, e.g. by booting from a CGB boot ROM. I think I'd rather add support for boot ROMs than hardcode the DMG-on-CGB palette initialization logic, because it's pretty involved (see SameBoy's open source boot ROM).

There are also some hardware edge cases that are different between DMG and DMG-on-CGB. I think the most noticeable are the STAT bug being fixed on CGB and APU wavetable RAM having different initial contents at power-on.