Emulate EGA/VGA PALETTE + CGA Background

[flagxor]

Currently CGA/EGA/VGA are emulated by keeping a display buffer in 32-bit RGBA.
A color_map is used to track mode colors.
Correct palette handling would work better if the display was stored in mode colors and converted on each from to RGB.

[flagxor]

GORILLAS.BAS uses a custom EGA palette, so requires this.