Implement the mode parameter of GCOL for drawing commands

- 0: Plot the specified colour
- 1: OR the colour with the colour already there
- 2: AND the colour with the colour already there
- 3: XOR the colour with the colour already there
- 4: NOT (invert) the colour already there
- 5: No change to colour already there
- 6: Clear the colour - set it to colour 0

NB: FabGL only supports NOT and swapping the foreground and background colours