Multicolour
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Mode 0: 256 colors (136 at the same time)
Mode 1: 10 colors
Mode 2: 4 colors

RSX comands:
|page,1       show screen 1
|page,2       show screen 2
|page,3       show screen 1 and 2 at the same time
|ssave,@a$    save compressed screen to file a$
|sload,@a$    load compressed screen from file a$
|pen,x,y      choose pen color x (0-255) with brightness y (0-1)
                  where y is only relevant for mode 2, otherwise should be 0

Color calculation:
- Check the color table in your handbook
- e.g. to mix red (3) with magenta (7) in mode 0:
  max(3,7)*16 + min(3,7) = 115
- |pen,115,0

NOTE:
- ink command works as usual
- normal pen command does not work any more
- "symbol after" command requires an address higher than 4000
- CPC664/6128: Mode 0 is a little bit disarranged, call &bb4e to fix this
