Support the composite palette for PC Engine

[einstein95]

As described on ​https://www.retrorgb.com/pc-engine-palette-improvements-the-amazing-people-behind-the-technology.html the PC Engine uses an inbuilt lookup table when converting the RGB to composite. I've used the palette file from ​https://github.com/Kitrinx/TG16_Palette/raw/main/palette.pal with Mednafen to generate the following comparison screenshot (ScummVM on left, Mednafen+composite palette on right)