Add VDU4 and VDU5 support about agon-vdp

[breakintoprogram]

Should be a straightforward fix:

• Add a flag that is set on VDU 5, cleared with VDU 4, MODE - done
• When printing a character, check the flag and use one of two print character routines - done
• When printing to graphic, only clear character space when deleting - done
• All cursor operations work but no scrolling - done

[breakintoprogram]

Note that printing character 0x7F (DEL) will do a correct backspace/delete, but the MOS editor does not use that character when deleting text on screen. So backspace when editing will not delete characters.