Screen handling

[larsbrinkhoff]

RGB and monochrome output goes to different pins. So screen_draw needs to be split in two: _rgb, and _mono. Only one should be active at one time. Which one depends on the monitor_sm124 flag.

Why does this matter? E.g. temporarily switching from colour to mono should result in some black pixels on the screen.

[larsbrinkhoff]

Also, the BLANK signal should disable RGB output.

But according to schematics, the GLUE BLANK signal should not affect the monochrome output signal. On the other hand, @troed claims there is some blanking going on. As well it must, or else the retrace should be visible on the monitor. More research needed. Possibly, the shifter blanks the signal itself?

[larsbrinkhoff]

Added BLANK in #86.

[larsbrinkhoff]

BLANK could be used to calibrate the width of the screen window, and the Vsync signal could be used for th window height.

[larsbrinkhoff]

It may or may not be a good idea to allow two connected screens: one colour and one monochrome. That would open two windows.

The -C command line option should be used to specify a colour screen. Including both -C and -M would connect two monitors.