MODE 7 support in Brandy BASIC V/VI
===================================
As of version 1.21.2, MODE 7 is now running using a text buffer,
which recalculates the line whenever a character in it is changed. This
allows for existing lines to be modified with control codes, and existing
characters are changed to reflect the modification.

This is a change to the teletext in 1.21.0 and .1, where streams of characters
were obeyed and displayed (mostly) correctly, but placed characters would not
reflect existing codes, nor would placed codes affect existing characters.

Glyphs are using 16x20 character cells, this allows for a decent rendering
of mosaic graphics, especially separated graphics. Two fonts are used,
the main one is the high-res font based on the one from RISC OS 5, the other
being the classic SAA5050 font.

Direct screen memory access is implemented at addresses &FFFF7C00 to &FFFF7FFF.

Some RISC OS 5 extensions are also supported:
  (source: http://beebwiki.mdfs.net/VDU_23)
VDU 23,18,1,flags,0,0,0,0,0,0 - Set Teletext display update
  bit 0 - suspend immediate updates of display bitmap (default: unset)
  bit 1 - update display after every character (default: set)
  
  Note: The defaults are the most accurate, but slowest setting. For
        performance, unset bit 1 first, then set bit 0 for the maximum
        performance at the expense of screen updates.

VDU 23,18,2,flags,0,0,0,0,0,0 - Set Teletext reveal state
  bit 0 - if set, any concealed text will be displayed. If cleared, concealed
          text is concealed.
Setting or clearing this will cause an immediate refresh of the display.

VDU 23,18,3,enable,0,0,0,0,0,0 - Set Teletext black enable
  enable = 0: Control codes &80 and &90 do nothing (default)
  enable = 1: Control code &80 selects alphanumeric black,
              control code &90 selects graphics black.
Enabling or disabling this will cause an immediate refresh of the display.

A Brandy-specific extension:
VDU 23,18,255,width,0,0,0,0,0,0 - Set Teletext character cell width and font.
  Valid values are 12 - 17. All other values are ignored.
  Bit 0 sets the font, if set use the classic SAA5050 font, if not set use
  SAA5050 for width 12, and the RISC OS 5 font for 14 and 16.
