Add control for NTSC artifacting

[craigthomas]

With certain graphics modes, NTSC artifacts generate more colors than were actually available on the Color Computer series. This is the case for the G6R mode, where only two color choices were available - black and white. With NTSC displays, the timing of the screen refresh led to certain pixel combinations of alternating black and white patterns to be displayed as red and blue instead. The emulator currently supports displaying NTSC artifacts when in this mode as a fat pixel approximation of how they would appear. However, it is possible to turn this behavior off entirely so that only the black and white pixels are displayed. The purpose of this feature request is to add a control to the emulator window that will allow the user to switch this behavior.