read colours

[stevesims]

reads colours, either from the palette, or one of the active colours

this is done using VDU 23, 0, &94, colour. values will be returned using the PACKET_SCRPIXEL response packet, and therefore MOS will interpret these exactly the same as reading a screen pixel via VDU 23, 0, &84. values will therefore be exposed in MOS in the exact same scrpixel and scrpixelIndex sysvar bytes

when using this command, colours 0-63 will be interpreted as palette colours, 128 will return the currently selected text foreground colour, 129 the text background colour, 130 graphics foreground, and 131 graphics background. any other colour values will be ignored

addresses #145