GPGX colours don't match hardware or GPGX in OpenEmu

[YoshiRulz]

(Where OpenEmu can be considered "upstream" for this core as it doesn't have its own frontend.)

Left: EmuHawk, right: OpenEmu

Initially reported by @alk in the TASVideos Discord, who also verified against hardware.

[jmredfern]

Bizhawk might be using a linear palette instead of the nonlinear palette used by the original hardware
FYI the ROM I used is here: https://ftp.untergrund.net/users/irokos/titan/titan-mdmodplayer.zip

[jmredfern]

@YoshiRulz You can find exact values about the Genesis nonlinear palette here which can be used to produce perfect color accuracy output: https://gendev.spritesmind.net/forum/viewtopic.php?t=2188

[YoshiRulz]

@Morilli Can you look into this?