M7PLOT - Teletext MODE 7 PLOT support
		    =====================================
			  J.G.Harston - 12-Apr-1994

M7PLOT installs a VDU extension driver to provide PLOT support in MODE 7
using teletext block graphics characters. It supports PLOT commands 0 to 15,
called by DRAW and MOVE, and 64 to 71 to plot points. As CLG is not passed
to the VDU extension vector, PLOT &80+n,0,0 is used to select the specified
graphics colour and clear the screen.

Syntax: *M7PLOT (ON|OFF) - defaults to ON

PLOT actions supported are:
    %xxxxxx00 - move
    %xxxxxx01 - set pixel
    %xxxxxx10 - invert pixel
    %xxxxxx11 - unset pixel
    %xxxxx0xx - relative coordinates
    %xxxxx1xx - absolute coordinates
    %xxxx0xxx - include last point
    %xxxx1xxx - exclude last point
    %00xxxxxx - draw line
    %01000xxx - plot point
    %10000xxx - clear screen with colour

The MODE 7 display is a multiple of 10 text columns, so it has 1280 (40*8*4)
horizontal logical graphics coordinates, 0 to 1279. MODE 7 characters are 10
raster lines high, so the display has 1000 (25*10*4) vertical graphics
coordinates, 0 to 999.

Graphics coordinates are -32768 to +32768 with (0,0) to (1279,999) within
the visible screen. Due to rounding, off-screen X coordinates are limited to
-1408 to +2687 and Y coordinates to -1173 to +2239. This gives just over a
whole screen off-screen in all directions.