Implement Additional Pixel Colors for Bitplanes

[craigthomas]

Currently, the emulator only defines 2 pixel colors - a color for pixels that are off and a color for pixels that are on. With the addition of bitplanes, there are now four colors possible:

• No pixels on
• Pixel on in the first bitplane
• Pixel on in the second bitplane
• Pixel on in both bitplanes

Here we see there needs to be 4 total pixel colors available. The purpose of this issue is to implement the additional colors in the emulator. For use when the bitplane functions are updated to make use of the new bitplane selector.

[craigthomas]

Implemented by PR #40